The move to criminalize being in the U.S. illegally would build on the Trump administration’s efforts to toughen immigration laws. Previously, immigrants in the country illegally were committing a civil offense and could be detained and deported, but weren’t considered to have committed a crime.

“Aliens in this country illegally face a choice,” Homeland Security Secretary Kristi Noem wrote in a memo describing the new policy. “They can return home and follow the legal process to come to the United States or they can deal with the consequences of continuing to violate our laws.”

The administration’s plans rely on obscure provisions of immigration law that have proved impractical to enforce in the past. Pursuing cases against alleged offenders could tie up prosecutorial resources and swell the incarcerated population, legal experts said. Immigrants in the country illegally rarely have the means to pay such steep fines, the experts said.

A law passed in 1940 created an immigrant registry to catch suspected communists. For years, the government aired television ads reminding all immigrants, including permanent residents, to register annually at local post offices. The system fell into disuse by the 1960s, when the government decided it was too costly and provided little benefit.

President Trump’s re-established registry requires anyone in the country illegally who hasn’t interacted with the government, such as by applying for asylum or a work permit, to come forward. The administration created a registration form and gave people 30 days to complete it, the documents show.

Immigrants who overstay their visas or are in deportation proceedings wouldn’t need to register, as the government already has their information, though they would be required to submit any changes of address.

Trump ordered the government to set up some kind of registration process in one of his day-one executive orders. The next day, the Justice Department ordered U.S. attorneys’ offices to give priority to prosecuting cases involving an immigrant’s failure to register. 

Then-President George W. Bush created a similar immigration registry after the Sept. 11, 2001, terrorist attacks, when his administration required men and boys from predominantly Muslim countries to submit photographs and fingerprints to the federal government. Tens of thousands of people who registered under that program were arrested and deported.