On Thursday evening, lawyers helping Venezuelan immigrants most at risk of being removed under an 18th-century wartime powers act received an ominous alert: U.S. immigration officials were handing out notices at a detention facility in Texas, informing migrants that they were considered enemies under the law and would be removed from the country.

“I am a law enforcement officer authorized to apprehend, restrain and remove alien enemies,” read the notice, a copy of which was filed in federal court by the American Civil Liberties Union. “Accordingly, under the Alien Enemies Act, you have been determined to be an alien enemy subject to apprehension, restraint and removal from the United States.”

The notice said the migrant could make a phone call but did not specify to whom. The single-page notice also did not mention any way to appeal the order.

The Supreme Court ruled this month that migrants must receive advance notice that they are subject to removal under the rarely invoked wartime powers law — and that they must have an opportunity to challenge their removal in court.

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News of the notices being handed out at the Bluebonnet Detention Facility in Anson, Texas, warning of impending deportations prompted a flurry of legal actions by the A.C.L.U. on Friday in several courts. Early Saturday, the Supreme Court stepped in with unusual speed, ruling that no flights could depart.

“The government is directed not to remove any member of the putative class of detainees from the United States until further order of this court,” the court said. It is unclear when the justices will make a ruling on whether deportation flights can continue.

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