Ali Saleh Kahlah al Marri -- an admitted Al Qaeda operative in the United States -- was released last week from a federal prison prior to completing his 15-year sentence because of “time served,” the Justice Department told Fox News on Tuesday.

Al Marri had been in U.S. custody since 2001, after reportedly being picked up on a routine traffic stop just weeks after the 9-11 terror attacks.  During a routine check, police discovered federal authorities already had him on their radar.

He was arrested and charged with providing "material support or resources" to 9-11 mastermind Khalid Sheikh Mohammed and other Al Qaeda operatives.

The 49-year-old al Marri, a Qatar native, at the time of his arrest was a U.S. resident attending graduate school.

He was declared an enemy combatant in 2003 and sent to a Navy brig in Charleston, S.C.

In 2008, U.S. courts ruled that al Marri, as a U.S. resident, was entitled to a federal hearing. He accepted a plea deal in 2009 that included the 15-year sentence, reportedly at a federal prison in Illinois.

Al Marri was freed on Friday, according to the Federal Bureau of Prisons.

His departure follows a series of releases of detainees from the U.S. prison in Guantanamo Bay, Cuba, as President Obama tries to close the facility, opened in the wake of the 9-11 attacks.

The recent Guantanamo releases have drawn sharp reaction from critics who say the president is now freeing the last and most dangerous detainees, who pose a high risk of re-establishing terror group connections.  

Al Marri reportedly has returned to Qatar as part of a repatriation agreement.

The country last year accepted five Guantanamo Bay detainees, released in exchange for the Taliban’s release of U.S. Army Sgt. Bowe Bergdahl.

As part of the deal, the Qatar government imposed a one-year travel ban on the detainees and pledged they would pose no further threat to the U.S.

According to the terms of al Marri's 2009 deal, he admitted to conspiring to "provide material support or resources" to Mohammed and other al Qaeda operatives.

“The defendant will plead guilty because he is in fact guilty,” federal prosecutors said in the court documents.

In a 2007 speech at the U.S. Coast Guard Academy, in Connecticut, President George W. Bush acknowledged that al Marri was taken into federal custody because of the danger he posed to Americans.

"Our intelligence community believes (he) was training in poisons at an al Qaeda camp in Afghanistan, and had been sent to the United States before September the 11th to serve as a sleeper agent ready for follow-on attacks," Bush said. “Among the potential targets our intelligence community believes this al Qaeda operative discussed … were water reservoirs, the New York Stock Exchange and United States military academies such as this one.”