Trump's use of the Alien Enemies Act of 1798 is under scrutiny. Here's what to know.

President Trump's invocation of the Alien Enemies Act of 1798 to deport Venezuelan migrants suspected of being members of the Tren de Aragua gang quickly kicked off a legal battle.

A federal judge issued a temporary restraining order that blocked the use of the 18th-century law. However, on the same day as the March 15 order, three flights carrying more than 200 Venezuelan men were flown to El Salvador to be held in the nation's maximum security prison. 

White House officials told CBS News last week that 137 of those Venezuelans were expelled under the Alien Enemies Act. Another 101 were removed under what officials described as "regular" proceedings under the 1952 Immigration and Nationality Act, including people who allegedly signed deportation papers after crossing the border illegally, officials said.