A Qatari citizen who was held in the United States since 2001 and declared an "enemy combatant" has been released and returned home.

Ali bin Kahlah al-Marri, who is a US resident, was arrested shortly after the 9/11 attacks and initially held on charges including credit card fraud.

The government later argued that he was a "sleeper agent" for al-Qaeda and he was held in solitary confinement for six years after being declared an "enemy combatant" and put in military custody.

His lawyer Andrew J Savage told Al Jazeera that al-Marri had been held under "extreme circumstances" from June 2003 until October 2004 without counsel or access to anyone outside the high-security facility where he was being held.

He "experienced enhanced interrogation during that time including dry-boarding", the lawyer said. Dry-boarding is a torture method that induces the first stages of asphyxiation by stuffing the detainees’ airways with rags, then taping shut his mouth and nose.

During the George W Bush era, the designation "enemy combatant" allowed the US to hold “terror suspects” at length without criminal charges.

In 2009, al-Marri pleaded guilty to one count of conspiracy to provide material support to a terrorist organisation.

Savage said al-Marri had not been planning an attack and had pleaded guilty so that he could serve his sentence and then be reunited with his wife and his five children in Qatar.

He was sentenced to 15 months in prison, but the judge agreed to consider the time he had already served.

Al-Marri had been slated to be released on Sunday and deported to Qatar. He was freed two days early and arrived in Doha on Saturday, his family said.

He was the last "enemy combatant" to be held on US soil.