After a Honduran immigrant arrived in the United States in 2022, officials ordered him to use a government-issued app as part of an immigration surveillance program.

At least once a week, the immigrant, a former police officer in Honduras who was living in Louisiana, would take a selfie through the facial-recognition powered app to confirm his identity and location. By trading some of his privacy, he avoided being put in a detention center and obtained a work permit.

In February, he received a message: report to an immigration office so the tracking technology could be updated. When he arrived, federal agents were waiting. They handcuffed him and put him on a vehicle bound for a detention center, where he has been ever since, according to an account from his wife and Jacinta González, the head of programs for the advocacy group MediaJustice who is working with the detained immigrant. He and his wife declined to be named for fear of harming his legal proceedings.

The maker of the app he had used was Geo Group, the largest private prison operator in the United States. Over the past decade, the company has also built a lucrative side business of digital tools — including ankle monitors, smart watches and tracking apps — to surveil immigrants on behalf of the federal government.

Those products are now aiding President Trump’s deportation efforts by providing the whereabouts of unauthorized immigrants to Immigration and Customs Enforcement, according to legal aid groups and immigration organizations. No figures have been released about the number of arrests made from the digital monitoring program, but legal aid groups estimated it was at least in the hundreds. More than 30,000 immigrants were arrested in Mr. Trump’s first 50 days in office, according to the Department of Homeland Security.

“These are the people who are precisely being monitored,” said Laura Rodriguez, a lawyer with American Friends Service Committee, a legal aid organization in New Jersey with several clients in the monitoring program who were detained. “It’s just easy pickings.”

The use of Geo Group’s technology has made the company one of the Trump administration’s big business winners so far. Even as Mr. Trump slashes costs across the federal government, his agencies have handed Geo Group new federal contracts to house unauthorized immigrants. And D.H.S. is weighing the renewal of a longtime contract with the company — worth about $350 million last year — to track the roughly 180,000 people now in the surveillance program.

Republican lawmakers and administration advisers have also called for more surveillance of immigrants, including expanded location tracking and stricter enforcement of curfews.

Mr. Trump’s immigration policies have sent Geo Group’s stock price soaring and kept its share price afloat even as the stock market gyrates. While digital monitoring generates only about 14 percent of its $2.4 billion in annual revenue, the company, which is based in Boca Raton, Fla., has said its immigrant surveillance could more than double. Profit margins on the monitoring business hover at around 50 percent.

“The Geo Group was built for this unique moment in our country’s history and the opportunities that it will bring,” George Zoley, the company’s founder, said on an investor call days after Mr. Trump was elected.

The tracking program that Geo Group oversees, called Alternatives to Detention, was set up to keep tabs on unauthorized immigrants who face potential deportation. Rather than being placed in detention centers or released into the country without supervision, immigrants receive location tracking devices. They must quickly respond to alerts sent to the gadgets in order to confirm their whereabouts, or risk punishment.

The program highlights technology’s growing role in guarding borders, with demand for muscular digital tools opening lucrative avenues for private industry while expanding government authority. The boom has benefited companies like Palantir, Anduril and Cellebrite, which have won government contracts.

Supporters praised the effectiveness and cost savings of Geo Group’s tools, but critics warned that the technology usage might lead to deeper surveillance of immigrant communities.

“The government bills it as an alternative to detention,” but “we see it as an expansion to detention,” said Noor Zafar, a senior lawyer with the American Civil Liberties Union.

At the same time, Geo Group’s products have been glitchy and expensive, according to more than a dozen current and former employees and government officials, as well as a review of the company’s federal contract and other records.

Each time an immigrant sends a selfie to check in through the company’s SmartLink app, which can happen millions of times a year, the federal government pays roughly $1, according to portions of Geo Group’s government contract obtained by The New York Times. The company charges $3 a day for any immigrant wearing its VeriWatch smartwatch. If the watch is lost, Geo Group bills the government $380, more than the cost of an Apple Watch SE.

ICE said in a statement that the monitoring program “effectively increases court appearance rates and compliance with release conditions.” The White House did not respond to requests for comment.

Attempts to modify the program and open the contract to rival bids have been stymied by Geo Group’s lobbying and connections on Capitol Hill and within ICE, according to senior D.H.S. officials and congressional staff members. Some senior ICE employees have gone on to work at the company.

Geo Group referred questions about how its monitoring technology is being used by the Trump administration to ICE. In a statement, Geo Group said it had “never advocated for or against, nor have we ever played a role in setting immigration enforcement policies.” The company added that its services are “closely monitored in accordance with strict government contract standards.”

From Prisons to Surveillance

Mr. Zoley, whose family moved to the United States from Greece when he was a child, started Geo Group in 1984 as a division of a security guard business. When the prison population exploded in the 1980s, the company expanded into running private prisons. It now has about 100 facilities.

In 1986, Geo Group won an ICE contract to build an immigrant processing facility in Aurora, Colo., to hold up to 150 people. By the 2000s, immigration had become a major business, which fluctuated based on who was in the White House and which party controlled Congress.

To diversify, Geo Group turned to digital surveillance. In 2011, the firm paid $415 million for Behavioral Interventions, a Colorado company founded in the 1970s to track cattle and which had expanded to monitoring parolees. Behavioral Interventions had an exclusive contract with ICE to digitally monitor thousands of recently arrived immigrants.

Mr. Zoley called the acquisition “transformative.” He was proved right when the government plowed hundreds of millions of dollars into remote surveillance of immigrants over the next decade, especially during the Biden administration.

The idea was that remote surveillance of immigrants facing removal proceedings would reduce the burdens on already-packed detention centers, relieve ICE officers of grunt work and save money. Digitally monitoring an immigrant costs about $4.20 a day, versus about $150 a day in a detention center, according to ICE.

“The program is meant to make sure we know who these people are and that they are on an adequate level of supervision,” said Deborah Fleischaker, the ICE chief of staff during the Biden administration.

By 2022, more than 300,000 immigrants were enrolled in the program. Geo Group’s sales soared, but revenue fell in 2023.

The company lobbied to expand the surveillance, said Jason Morín, a political science professor at California State University, Northridge who studies Geo Group. Ahead of the 2024 election, a Geo Group subsidiary gave more than $2 million in campaign contributions to Republican candidates, with the bulk going to groups supporting Mr. Trump and those running for Congress, according to Federal Election Commission records.

Wall Street analysts included Geo Group, which has about 18,000 employees, in ideas for stocks that would perform well if Mr. Trump were elected. With no real competition, some estimated the company’s digital monitoring business would generate nearly $700 million in revenue cumulatively through 2026. Its biggest shareholders include BlackRock and Vanguard.

Living in a Radius

For many unauthorized immigrants who are not detained at the border, the perilous journey to the United States ends inside Geo Group’s surveillance system.

After turning themselves in to immigration officers, they are given an ankle bracelet, a smartwatch or a smartphone with the company’s monitoring app. Rather than be overseen by ICE officers, they are watched by Geo Group case specialists.

Under the program, immigrants live more freely in the United States during a legal process that can play out over years. The trade-off is constant monitoring. Geo Group’s app has permission to continuously track a user’s location, according to a Times analysis of its code.

One Geo Group case worker in the Northeast, who declined to be identified for fear of retaliation, described using a Google Maps-like software to check immigrants’ locations. If immigrants were not home or lied about their whereabouts during a check-in, they received a strike. If an immigrant received three strikes, the case specialist would inform an ICE agent, who could increase monitoring, detain the person or expedite the person’s deportation.

Geo Group employees at field offices from Massachusetts to Alabama said they had often struggled to monitor up to 300 immigrants simultaneously. The case worker in the Northeast recalled being asked to make 12 home visits to immigrants in a single day. Each was limited to five minutes, despite requirements to do a full report on the immigrant’s living conditions, she said. Geo Group charged D.H.S. up to $88 a visit.

Those under surveillance are limited in where they can travel, lawyers and immigration rights groups said. If immigrants leave a set area of where they can be, the software alerts case officers. Because many check-ins must happen from home on a prescribed day — say on a Friday from 9 a.m. to 5 p.m. — people are often stuck waiting, affecting their ability to work or perform certain day-to-day tasks.

“Whatever radius is imposed, that becomes the size of their life,” said Laura Rivera, a senior lawyer for Just Futures Law, which focuses on tech usage for immigration enforcement.

Geo Group stores data collected from the surveillance program on its private servers, making it more cumbersome for the government to access and analyze, current and former ICE officials said. Former company employees described technical problems, such as relying on outdated servers that frequently crashed, weak batteries in the company’s smartwatches and a bug in which the app occasionally failed to tell an immigrant to check in, which could result in a penalty.

Keeping the Contract

In 2022, as Geo Group’s digital monitoring business ballooned, some Biden administration officials in the Department of Homeland Security questioned the cost and effectiveness of the tracking program.

The D.H.S. officials met to draft a plan to change it, including standards for assessing each immigrant’s risk of committing a crime or fleeing and what surveillance that merited, said six people familiar with the conversations who requested anonymity in order to discuss internal deliberations. The officials wanted to break up the contract into three parts to solicit new bids, the people said. Around the same time, D.H.S. tech workers were asked to develop cheaper alternatives to Geo Group.

​The moves threatened Geo Group’s involvement in the monitoring program, with major financial implications for its bottom line. The company began lobbying to disrupt the plans, according to agency officials and Capitol Hill staff members.

Conservatives and some career ICE officials joined in. Thomas D. Homan, who was then working for a conservative immigration group and is now Mr. Trump’s border czar, wrote a Breitbart editorial attacking the plans and the midlevel Biden administration official responsible for them. A conservative group created a website dedicated to attacking the official.

Daniel Bible, ICE’s head of enforcement and removal operations at the time, also stalled the changes by ordering lengthy reviews and delaying approvals, two people said. Last year, he joined Geo Group as an executive. He did not respond to a message for comment.

The efforts eventually died and plans to develop cheaper alternatives to Geo Group’s tech never went beyond testing.

Geo Group said allegations that it had blocked changes to the surveillance program “are part of a politicized effort by open borders groups to interfere with the federal government’s immigration enforcement efforts and to abolish immigration enforcement writ large.”

Since Mr. Trump took office, fewer immigrants have crossed the border as the president has signed legislation like the Laken Riley Act, which mandates increasing detentions of immigrants with criminal histories in facilities like those owned by Geo Group.

The new law could also require “significant ramp-up in the electronic monitoring,” Mr. Zoley said on an earnings call in February, adding that his company was ready to scale up its surveillance “by several hundreds of thousands and upward to several millions of participants as required.”

Geo Group’s technology has repeatedly helped ICE officers carry out deportations, legal aid groups said. In January, ICE agents in Georgia tracked an immigrant to a job site and detained him, while another was grabbed outside a church, the groups said. More recently, an immigrant in New Jersey received a call from a Geo Group employee asking him to step outside his home because the tracker was not getting a signal. Agents were waiting for him.

Legal aid groups said they feared that the surveillance would soon be used for larger raids. In 2019, during the first Trump administration, agents in Mississippi used data harvested from Geo Group’s tools to help secure a warrant for a raid on a chicken processing plant. The ensuing sweep, which included workplaces across the state, led to the detention of 680 immigrants.

During a 24-hour swing through Copenhagen last month, Eric Slesinger met with engineers making maritime drones, developers of war-planning software and an adviser to NATO. He had recently visited London for a dinner with a senior British intelligence official and would soon head to the Arctic to learn about the technologies that could handle extreme climates.

The packed schedule would seem more common for Mr. Slesinger in his former job as an officer at the Central Intelligence Agency. But now the 35-year-old was in high demand as he parlayed his spy agency credentials into a career as a venture capitalist focused on the suddenly relevant area of defense and national security technology in Europe.

“This is all happening at warp speed,” said Mr. Slesinger, who has backed eight defense start-ups and has negotiated with several more.

As President Trump throws the future of the trans-Atlantic relationship into question, governments across Europe have outlined plans to potentially spend hundreds of billions of euros on weapons, missile-defense programs, satellite systems and other technologies to rebuild their armies. Technologists, entrepreneurs and investors are racing to take advantage of the spending boom by creating new defense start-ups.

Few paid attention several years ago when Mr. Slesinger moved to Madrid with the idea that Europe would need to drastically increase defense spending because U.S. military protection could not be taken for granted. Now his predictions look prescient. After Mr. Trump’s inauguration, which followed his defeat of Vice President Kamala Harris in the November election, members of his administration called Europe “pathetic” and military mooches of the United States.

“Whether Trump won or Harris or anyone else, the fact would have remained that there’s a technology catch-up that needs to happen in Europe,” Mr. Slesinger said while walking between meetings in Copenhagen last month. “Maybe it’s accelerated in certain ways, but this was a long time coming.”

Mr. Slesinger is now in the unusual position of a former American intelligence officer who is trying to profit from Europe’s planned military transformation. His one-man venture capital firm, 201 Ventures, is completing a $22 million fund to invest in young start-ups at the intersection of tech and national security.

Mr. Slesinger’s initial investments include a maritime drone company in Sweden, a maker of manufacturing technology in Britain, an artificial intelligence firm in Greece and a hypersonic vehicle start-up in Germany.

The United States has a long tradition of investing in defense — Silicon Valley was started in part with Pentagon funding — and has seen the rise of several military-centric start-ups, such as Palantir and Anduril. Europe has had fewer successes, partly because defense-related businesses were viewed as so unethical that many investors there refused to put money behind them.

“There has been this awakening moment, and it’s going to result in a dramatic increase in spending in defense, security and resilience technology,” said Chris O’Connor, a partner at the NATO Innovation Fund, a 1 billion euro tech fund started with money from 24 members of the North Atlantic Treaty Organization, though not the United States.

The NATO fund is the biggest financial backer of Mr. Slesinger’s firm. Mr. O’Connor said Mr. Slesinger’s national security experience made him ideal for identifying companies with tech that could win government contracts.

“He’s going to end up playing a critical role,” Mr. O’Connor said.

Mr. Slesinger grew up just outside Washington, D.C., and attended Stanford. There, he was a standout in the mechanical engineering program, said Craig Milroy, co-director of Stanford’s Product Realization Lab, where students can workshop hardware ideas.

While many of Mr. Slesinger’s Stanford classmates explored jobs with Apple or Google, he looked elsewhere. “He came into my office one day and said, ‘I’m applying to join the C.I.A.,’” Mr. Milroy said. “That’s never happened before or since.”

Mr. Slesinger is cagey about his five and a half years working at the C.I.A. But with his engineering background, he said, he worked among more Q-like figures from the James Bond films, geeks operating in the background to solve technical problems for intelligence officers in the field.

“Imagine being a student, kind of nerd engineer, and then you get to go in this place where you have like a Santa’s workshop-like capability,” he said. “Intelligence problems are really hard, they’re gnarly, and you feel a real responsibility to do something to solve the problem.”

In 2019, Mr. Slesinger stepped back from the agency to attend Harvard Business School. He also spent a summer working for the C.I.A.’s venture capital fund, In-Q-Tel.

Around this time, he became fixated on the idea that Europe must rebuild its militaries after a generation of low investment. The United States spent about $880 billion on defense in 2024, more than double what other countries in NATO spent combined.

With the United States focused on China, Mr. Slesinger was convinced he would see the end of the so-called peace dividend, which has allowed European countries to spend more on social services and pensions since World War II, instead of on tanks and fighter jets.

Russia’s invasion of Ukraine in 2022 further crystallized his thesis. He then started the European Defense Investor Network, which now includes about 125 investors, entrepreneurs and policymakers. Last year, he started 201 Ventures.

At first, he struggled to raise funding because many investors refused to back military technologies. But he eventually raised money from NATO and found advisers including Eileen Tanghal, who used to oversee In-Q-Tel’s London office; David Ulevitch, a general partner at the Silicon Valley venture firm Andreessen Horowitz; and the author Sebastian Mallaby.

Over the past 12 months, Mr. Slesinger, who also has an Italian passport from his family’s roots there, has traveled to 15 countries. On a recent trip to the Arctic, he rode a snowmobile to a remote area being considered for testing new power sources and a communication technology. In Switzerland, he toured the world’s most powerful particle accelerator.

In February, Mr. Slesinger was in Germany for the Munich Security Conference when Vice President JD Vance delivered a blistering speech criticizing Europe. Within weeks, Germany, France, Britain and other European countries pledged to vastly increase military spending, alarmed that they could no longer count on the United States as a reliable ally.

“It felt like a sea change,” said Mr. Slesinger, who watched Mr. Vance’s speech from a laptop at a nearby hotel. “You could feel it as he was speaking.”

How much of the new spending will reach start-ups is unclear. Missiles, ammunition and fighter jets are likely to be higher priorities than tech from small, untested companies.

Mr. Slesinger said it would take years to measure success, but he expects to spend his $22 million fund in the next two years and has already begun thinking about raising a larger amount. In the past few months, he has been peppered with pitches from European entrepreneurs suddenly interested in making military tech.

For just about everyone he meets in Europe, there is one nagging question: Is he really no longer working for the C.I.A.?

“I’m really out!” he said.