Washington DC - A federal judge on Friday ordered the return to the US of a Salvadoran migrant who was mistakenly deported last month to a notorious prison in El Salvador.

Kilmar Abrego Garcia (29), who was living in the eastern state of Maryland, was among a group of undocumented migrants who were flown to El Salvador by the Trump administration on March 15.

Justice Department lawyers admitted in court filings that Abrego Garcia, who is married to a US citizen, had been deported due to an "administrative error."

District Judge Paula Xinis, at an emergency court hearing on Friday, said Abrego Garcia was taken into custody "without legal basis" on March 12 and deported three days later "without further process or legal justification."

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Xinis ordered his return to the US no later than 11:59 PM on April 7.

"His continued presence in El Salvador, for obvious reasons, constitutes irreparable harm," the judge said in her order to the Department of Homeland Security and other agencies.

Three planeloads of undocumented migrants were flown to El Salvador on March 15 as part of President Donald Trump's crackdown on illegal immigration.

The Trump administration alleged that most of the deportees were members of the Venezuelan gang Tren de Aragua and it invoked the little-known 1798 Alien Enemies Act to justify their summary removal.

Attorneys for several of the deported migrants have said that their clients were not Tren de Aragua members, had committed no crimes and were targeted largely on the basis of their tattoos.

Abrego Garcia had been living in the US under protected legal status since 2019, when a judge ruled he should not be deported because he could be harmed in El Salvador.

The White House, citing unreleased evidence, insisted earlier this week that Abrego Garcia was a member of the Salvadoran gang MS-13.

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"The administration maintains the position that this individual who was deported to El Salvador and will not be returning to our country was a member of the brutal and vicious MS-13 gang," White House Press Secretary Karoline Leavitt said.

Another district judge, James Boasberg, has barred the Trump administration from carrying out further deportations under the Alien Enemies Act, which has only been used previously during the War of 1812, World War I, and World War II.

The Trump administration has used images of the alleged gang members being shackled and having their heads shaved in the El Salvador prison as proof it is serious about cracking down on undocumented immigration.