President Trump late Monday announced on Twitter that U.S. Immigration and Customs Enforcement will begin the process of “removing the millions of illegal aliens who have illicitly found their way into the U.S.,” but did not elaborate on what new measures will be taken.

“They will be removed as fast as they come,” Trump said.

Mark Morgan, the director of the agency, did not announce any new initiatives during his stop in Louisville on Sunday, where he spoke about the humanitarian and national security crisis at the border.

ICE did not immediately respond to an email from Fox News for comment.

An administration official said that the new effort would focus on the more than 1 million people who have been issued final deportation orders by federal judges but remain at large in the country.

“Countless illegal aliens not only violate our borders but then break the law all over again by skipping their court hearings and absconding from federal proceedings. These runaway aliens lodge phony asylum claims only to be no-shows at court and are ordered removed in absentia,” the official said. “... These judicial removal orders were secured at great time and expense, and yet illegal aliens not only refuse to appear in court, they often obtain fraudulent identities, collect federal welfare, and illegally work in the United States. Enforcing these final judicial orders is a top priority for Immigration and Customs Enforcement.”

Some in Trump’s administration believe that decisive shows of force — like mass arrests — can serve as effective deterrents, sending a message to those considering making the journey to the U.S. that it’s not worth coming.

Earlier this month, Trump announced that the U.S. reached a deal with Mexico that includes plans to return migrants seeking asylum to Mexico, where they will remain until they can be processed.

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Trump praised Mexico in the tweet, saying the country has been doing a very good job at stopping those trying to gain access to the U.S. border.

The Associated Press contributed to this report.