More than 20,000 undocumented immigrants were arrested during the first month Donald Trump was president, according to the Department of Homeland Security.

But lawyers and immigrant advocates warned that the administration’s plan was on shaky legal ground and that some immigrants may be afraid to comply.

Immigrants in the U.S. illegally who fail to register with the federal government could face significant fines, potential imprisonment or both, according to a directive issued by the Department of Homeland Security on Tuesday.

The Trump administration is threatening to punish some immigrants in the U.S. illegally with criminal penalties, including jail time and fines, if they fail to sign up for a government registry.

Under new rules, all undocumented individuals who are 14 or older will need to register with the federal government and provide their fingerprints.

More than 20,000 undocumented immigrants were arrested during Donald Trump’s first month as president, according to the Department of Homeland Security and CNN. The Trump administration launched an immigration crackdown in the moments after his inauguration and publicized daily arrest totals during his first couple of weeks in office.