The two-page statement was sent Wednesday to the Inspector General at the Department of Defense by a high-ranking Marine Corps defense lawyer. A Guantanamo spokesman said the base would cooperate with any Pentagon investigation.
The lawyer sent the statement on behalf of a paralegal who said men she met on Sept. 23 at a bar on the base identified themselves to her as guards. The woman, whose name was blacked out, said she spent about an hour talking with them. No one was in uniform, she said.
A 19-year-old sailor referred to only as Bo "told the other guards and me about him beating different detainees being held in the prison," the statement said.
"One such story Bo told involved him taking a detainee by the head and hitting the detainee's head into the cell door. Bo said that his actions were known by others," the statement said. The sailor said he was never punished.
Other guards "also told their own stories of abuse towards the detainees" that included hitting them, denying them water and "removing privileges for no reason."
"About five others in the group admitted hitting detainees" and that included "punching in the face," the affidavit said.
"From the whole conversation, I understood that striking detainees was a common practice," the sergeant wrote. "Everyone in the group laughed at the others stories of beating detainees."
The statement was provided to the AP on Thursday night by Lt. Col. Colby Vokey. He is the Marine Corps' defense coordinator for the Western United States and based at Camp Pendleton.
Vokey called for an investigation, saying the abuse alleged in the affidavit "is offensive and violates United States and international law." A call to the inspector general's office was not immediately returned.
"The mission of the Joint Task Force is the safe and humane care and custody of detained enemy combatants. Abuse or harassment of detainees in any form is not condoned or tolerated," Navy Cmdr. Robert Durand said. "We will participate fully with the inspector general to learn the facts of the matter and will take action where misconduct is discovered."
The sergeant said a man named Shawn, who screened detainees' mail, told her it was "not uncommon" for mailroom clerks to withhold mail until they decided to release it.