WASHINGTON, United States (AP) — Hundreds of thousands of young immigrants living in the country illegally willingly came out of the shadows and identified themselves to the Obama administration on the promise that they’d be safe from deportation and allowed to work.

Some may now regret that decision.

President-elect Donald Trump has promised to immediately scrap the program that protected these immigrants. If he does, it’s not clear whether he would take action against the more than 741,000 participants. But if he decides to pursue them, the government now has their addresses, photographs and fingerprints.

Twenty-year-old Nancy Villas was among the first to apply for the Deferred Action for Childhood Arrivals program in the summer of 2012, waiting in line hours at a sign-up site at Chicago’s Navy Pier. Since then she’s been working part time at a child care centre to pay for college classes. Now she’s worried she may eventually be forced to return to Mexico, a country she left when she was 9.

“I knew it was the only way to have better opportunities,” Villas said. “I took the risk without thinking that somebody would want to take it away.”

Trump made illegal immigration the cornerstone of his campaign, promising to build a massive wall along the Mexican border and deport millions of people living in the country illegally. Once he takes office, Trump can almost immediately rescind the promised protection and, with it, likely void the accompanying work permits.

But there is little to suggest that he would move swiftly to deport program participants. In a post-election interview with CBS’ “60 Minutes,” Trump said he would focus initially on criminal immigrants living illegally in the U.S. He said that could be about 2 million to 3 million people, though that figure is likely inflated.

Mark Krikorian, executive director for Center For Immigration Studies, said the fears of program participants may be overblown.

“Unless there’s a crime issue or something specific that’s going to draw attention to an individual, I can’t see how they’d be a priority,” said Krikorian, whose think tank describes itself as low-immigration, pro-immigrant.

President Barack Obama initiated the program to shield from deportation young immigrants, some of whom don’t even remember their native countries. It didn’t give the immigrants legal status, only “deferred action” — meaning they wouldn’t face deportation while they participated.

There was never a guarantee that it would last beyond Obama’s term as president. A former immigration official who helped craft the program, John Sandweg, said the White House and the Homeland Security Department considered the reality that a future president could end it. But at the time, he said, it appeared that revoking already-approved protections would be politically difficult.

“These are the kinds of kids you should bring out of the shadows,” said Sandweg, a former acting director of Immigration and Customs Enforcement. “I don’t think anyone envisioned a President Trump when this was created.”

Trump wasn’t subtle about his opposition to the program. He called it an “illegal amnesty” and promised to “immediately terminate” the program. And since winning office, Trump has said he will nominate immigration hardliner Sen. Jeff Sessions as attorney general. As he considers other Cabinet vacancies, Trump has met with Kansas Secretary of State Kris Kobach, who led his state’s court fight to prevent an expansion of the deportation protection plan.

When the program started, the Obama administration suggested that application files would not generally be used for enforcement efforts. U.S. Citizenship and Immigration Services addressed the concern in its published “frequently asked questions,” saying information would be shared with enforcement officials only if someone “meets the criteria” for deportation proceedings.

But revoking the deportation protection would make those young immigrants almost immediately eligible to face deportation.

Sandweg said going after participants would be a massive logistical undertaking that would only worsen backlogs in an already overburdened immigration court system where many people wait years for a final decision.

Adding about 750,000 to the court system “would do nothing for public safety,” Sandweg said.

Nonetheless, the mere prospect of that has prompted some Democratic lawmakers to ask Obama to protect these immigrants with pardons before he leaves office.

And advocates for the young immigrants have pledged to keep up their fight to win public and political support for overhauling immigration laws.

“We organised across the country, we shared our stories publicly and we came together. We took direct actions and held politicians accountable,” said Cristina Jimenez, executive director and co-founder of United We Dream.

Under a Trump administration, Jimenez said, that won’t change.