The allegations come from a Marine Corps sergeant, 23-year-old Heather Cerveny, who spent a week at the base in late September as a legal aide to a military lawyer representing detainees.
In a sworn affidavit filed with the Pentagon Inspector General, Sgt. Cerveny says she met several Navy prison guards at a club on the base where, over drinks, they described harsh physical abuse.
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"One sailor specifically said, 'I took the detainee by the head and smashed his head into the cell door,'" Sgt. Cerveny tells ABC News in an exclusive interview.
She says she was "shocked" to hear several guards from different parts of the camp speak openly of mistreating prisoners.
"Everyone in the group laughed at all their stories of beating detainees," she recalled. "None of them looked like they cared. None of them looked shocked by it."
One of the guards "was telling his buddy, 'Yeah, this one detainee, you know, really pissed me off, irritated me. So I just, you know, punched him in the face.'"
Sgt. Cerveny says the guards also talked about taking away detainees' privileges "even when they're being good" and denying their requests for water. In her affidavit, she states she was told "they do this to anger the detainees so they can punish them when they object or complain."
When asked why, she claims a guard named Steven told her it's "because he hates the detainees and that they are bad people. He stated that he doesn't like having to take care of them or be nice to them," she says in the affidavit.
Sgt. Cerveny says the guards told her they worked at Camps 5 and 6. When she asked one of the guards about the consequences of their actions, "He said nothing. Everyone in the group was laughing."
They stopped laughing when they found out she worked for a marine defense lawyer.
The sergeant's superior officer, Lt. Col. Colby Vokey, says in his opinion, "These are crimes. These are offenses. Serious abuse of power is what we're seeing here...This is not humane treatment."
He says Sgt. Cerveny was initially reluctant to report what she heard, but he told her to start writing up her affidavit on the flight home.
Sgt. Cerveny says, in the end, she felt a duty to report what she heard, even if she might be accused of betraying the ranks. "What it comes down to for me, morals and simple rights and wrongs. And I don't think a uniform changes that."
The investigation into these new allegations comes as the military and the White House insist any problems in the treatment of detainees at Guantanamo have long since been fixed.
President Bush has said, "Our troops can take great pride in the work they do at Guantanamo Bay, and so can the American people."
About 440 terror suspects from around the world are being held at Guantanamo, including some the Bush administration has designated as "high-value detainees."