Afghan police threatened the family of teenager Mohammed Jawad while he was undergoing interrogation at a Kabul police station, said army colonel Stephen Henley, the judge, in a three-page ruling.
Jawad, now facing trial by military commission, is accused of throwing a grenade inside an Afghan bazaar in December 2002, which wounded two U.S. soldiers and their Afghan interpreter. None was killed.
Henley found in the ruling that there was reason to believe Jawad was under the influence of drugs at the time of his capture and forced confession.
He also accepted the accused's account of how he was threatened, while armed senior Afghan officials allied with U.S. forces watched his interrogation.
"You will be killed if you do not confess to the grenade attack," the detainee quoted an interrogator as saying. "We will arrest your family and kill them if you do not confess."