FBI agents saw military interrogators use abusive tactics on prisoners at the naval base at Guantanamo Bay, Cuba, officials confirmed last night.
The account of incidents in 2002 involving foreign terrorism suspects held at the base was in a July letter from FBI counter-terrorism official Thomas Harrington, to Major General Donald Ryder, the Army's provost marshal.
Mr Harrington, who headed a group of investigators which visited the base, detailed incidents including one in which a female Army interrogator grabbed a male prisoner's genitals and bent his thumbs backward.
He also described a prisoner being menaced by a dog and placed into isolation, and an incident in which a detainee's mouth was covered with duct tape.
In his letter, Harrington referred to the incidents as examples of "highly aggressive interrogation techniques" and asked Maj Gen Ryder, the Army's senior criminal investigator, to take "appropriate action."
Mr Harrington wrote that the FBI told Pentagon lawyers in January 2003 about the abusive treatment, but that the matter had not been addressed.
"We take all allegations seriously and investigate each one fully," Army Brigadier General Jay Hood, commander of the Guantanamo prison, said in a statement.
"The appropriate actions were taken, and some allegations are still under investigation."
The Pentagon has denied that detainees have been tortured at Guantanamo.
The US military holds about 550 non US citizens at the base - nearly all without charges or access to lawyers. Most were caught in Afghanistan and many have been held at the base for nearly three years.
Some men who have been released from the prison have stated they were tortured there. The International Committee of the Red Cross has accused the United States of using tactics "tantamount to torture" on prisoners.