President Trump tweeted Monday evening that Immigration and Customs Enforcement will begin removing “millions” of undocumented immigrants next week. “Next week ICE will begin the process of removing the millions of illegal aliens who have illicitly found their way into the United States,” Trump tweeted. “They will be removed as fast as they come in.” The president’s reference to mass arrests is an apparent nod to a long-planned operation to arrest migrant families in a “blitz operation” across the U.S., The Washington Post reported. Such operations by ICE are usually kept under wraps. The U.S. and Mexico reached an agreement this month in which Mexico has agreed to take in Central American immigrants seeking asylum until their cases are heard in the U.S. There are approximately 12 million undocumented immigrants in the U.S., with the majority from Mexico and Central American countries.