Mohammed Jawad was 17 when he was arrested six years ago, U.S. officials say, but his lawyer says he was 14 and family members say he was 12, McClatchy Newspapers reported.
Jawad's conviction for throwing a hand grenade at two U.S. soldiers, wounding them, was rejected by a U.S. judge last month. The judge said his confession was obtained under torture.
Jawad was scheduled to meet with Afghan President Hamid Karzai before arriving at the Pul-i-Charki Prison outside Kabul for processing and release, McClatchy reported.
"It's still not over until he can walk free, but he is almost there" said Marine Maj. Eric Montalvo, an attorney for Jawad.
After the U.S. judge's ruling, the Justice Department first said it would seek new evidence, but eventually it filed no charges and the U.S. military withdrew its charges. McClatchy said even the Army prosecutor came to doubt the case, and was relieved of duty.
The report said the teenager was subjected to sleep deprivation at Guantanamo, and was held as an adult in facilities that segregated supposedly hard-line members of al-Qaida.
More than 540 detainees have departed Guantanamo since 2002 for other countries, the Justice Department said.
U.S. officials have said they plan to put more than 60 remaining prisoners on trial. Fewer than 250 prisoners are believed to remain in the detention facility.
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