WASHINGTON (Reuters) - A U.S. appeals court on Monday blocked President Donald Trump’s administration from immediately transferring an American citizen accused of being an Islamic State member from military custody in Iraq to an unidentified country, apparently Saudi Arabia.

The man, who holds dual U.S.-Saudi citizenship and whose name has not been released, was captured in Syria and had been held in Iraq without legal representation until the American Civil Liberties Union intervened on his behalf in U.S. federal court last October.

The administration was seeking to transfer custody of the man even though U.S. courts had not resolved the legal challenge to win his release. A three-judge panel of the U.S. Court of Appeals for the District of Columbia Circuit ruled 2-1 on Monday to uphold U.S. District Judge Tanya Chutkan’s ruling last month that had stopped the planned transfer.

The appeals court’s reasoning was unknown, as both its majority opinion and the dissent were issued under seal.

U.S. Department of Justice spokeswoman Kerri Kupec said the government is reviewing the decision.

Saudi Arabia is believed to be the country to which the United States wants to send the detainee, based on media reports and a reference in a court filing.

The ACLU filed a so-called habeas corpus petition on the man’s behalf, saying the United States had no legal right to detain a U.S. citizen without charges and that he had the right to challenge his detention in court. The ACLU has argued that his detention violates the U.S. Constitution’s promise of due process of law.

The man was caught last September by Syrian Democratic Forces, an alliance of militias fighting against Islamic State, before being handed over to the United States.

The United States said it has evidence that he signed up as an Islamic State foreign fighter in 2014 and entered Syria in January 2015, according to court papers. The detainee, who has a degree in electrical engineering, is married with one child, according to court papers.

The ACLU has said the Trump administration has presented no evidence that the man fought for Islamic state, adding that if it has evidence of criminality it should charge him in a U.S. court, where he has the ability to exercise his constitutional rights.