DOHA (REUTERS) - A Qatari man declared an enemy combatant following the Sept 11, 2001, attacks and imprisoned as an al-Qaeda operative has been freed from a US prison and is back in Qatar, his lawyer said on Sunday.

Ali al-Marri, who was convicted in 2009 of providing material support to al-Qaeda, was released from federal prison in Colorado in the last few days and is home in Doha, Qatar, Charleston attorney Andy Savage said.

Marri, who is in his mid-40s, was arrested by the Federal Bureau of Investigation in 2001 and detained as a material witness to the Sept 11 attacks. He was charged with credit card fraud and lying to the FBI and held for 18 months before the US government dropped the charges in 2003.

President George W. Bush then declared Marri an "enemy combatant" and he was held in a Navy brig at Charleston, South Carolina, for five-and-a-half years.

The Supreme Court agreed to hear a challenge to his indefinite military detention from Marri. Before the court heard the case, President Barack Obama in 2009 ordered him transferred to Justice Department custody to face charges.

Marri pleaded guilty that year to one count of providing material support and resources to a foreign terrorist organisation. He was sentenced to eight years in prison.

Marri entered the United States legally on Sept 10, 2001, and was arrested in December 2001 in Peoria, Illinois, where he was attending college.

Harri had been suspected of being an al-Qaeda agent sent by the militant group to disrupt the US financial system by hacking into bank computers.