WASHINGTON 

U.S. President Donald Trump's administration is reportedly using a relatively unknown immigration regulation enacted after the Sept. 11 terror attacks to detain a Palestinian, even after his 15-year prison sentence had ended.

Adham Hassoun, 56, a computer programmer living in Florida, was convicted by the U.S. of sending support to Islamist militants abroad. He served a sentence of a decade-and-a-half, according to a report Tuesday by The New York Times.

Hassoun was then granted a temporary release by a judge, however, the Trump administration used an immigration regulation from 2001 to designate him a security risk and detain him indefinitely.

"The government is saying it can hold someone solely because it claims he is a danger, without a justification like he’s a combatant in an armed conflict or has a mental illness that prevents him from controlling his own actions," Jonathan Hafetz of the American Civil Liberties Union told The Times. "This is a monumentally important issue courts have never addressed."

Washington is finding it difficult to deport Hassoun, who is stateless because after being born in Lebanon, the country refused to give him citizenship. While the Palestinian Authority has offered him refuge in the West Bank, Israel and Jordan have blocked that potential move, according to the newspaper.

Hassoun has sued the government since being detained, which the Times says is the latest in a series of challenges over when the government can label someone a terrorism risk to indefinitely detain them.

Prior to this case, the U.S. government only used this immigration regulation once, when the administration of then-President Barack Obama used it to justify the detention of Mohammed Rashed, who served a sentence for planting a bomb on a plane in 1982. Before the government was able to rule whether his indefinite detention was lawful, Mauritania accepted Rashed.