The detainee, Mohammed Jawad, was arrested in Afghanistan in 2002 on charges that he threw a grenade at a US convoy in the country.
His lawyers say he was just 12 at the time of his arrest, while the Pentagon says he was 16 or 17 when he was transferred to Guantanamo and declared an "enemy combatant."
In court filings, Justice Department lawyers said they were withdrawing that designation and would no longer contest in court a challenge to Jawad's detention at Guantanamo, the US naval base in southern Cuba where 229 detainees are still held.
Instead of providing fresh evidence against the detainee, they asked US District Court Judge Ellen Juvelle to withhold judgement on the detention challenge while they "expedite" a criminal investigation against Jawad, with a possible view to a US federal trial.
The government "will no longer treat petitioner as detainable under the Authorization for Use of Military Force (AUMF) as informed by the laws of war," the filing said.
"The Attorney General has directed that the criminal investigation... continue, and that it do so on an expedited basis."
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