Reversing itself, the Army said Tuesday that a G.I. was discharged partly because of a head injury he suffered while posing as an uncooperative detainee during a training exercise at Guantánamo Bay, Cuba.

The Army had previously said Specialist Sean Baker's medical discharge in April was unrelated to the injury he received last year at the detention center, where the United States holds suspected terrorists.

Mr. Baker, 37, a former member of the 438th Military Police Company, said he played the role of an uncooperative prisoner and was beaten so badly by four American soldiers that he suffered a traumatic brain injury and seizures. He said the soldiers only stopped beating him when they realized he might be American.

Bruce Simpson, Mr. Baker's lawyer, said his client is considering a lawsuit.