President Donald Trump’s administration is using federal prisons to detain some people arrested in its immigration crackdown.

The federal Bureau of Prisons is accepting into their facilities thousands of immigrants detained by the Homeland Security Department, a highly unusual move that raises significant legal and logistical questions.

In recent weeks, authorities have arrested people in at least three states and accused them of acting like they’re carrying out the work of U.S. Immigration and Customs Enforcement.