FBI Officials have repeatedly warned the U.S. military of the aggressive interrogation methods used in the prison at the U.S. naval base in Guantanamo Bay, Cuba, according to FBI memos released Thursday.

The memos said those aggressive methods were legally risky and also likely to be ineffective.

One memo also said a senior officer at the prison for interrogating terror suspects have misled his superiors at the Pentagon that the FBI had endorsed the aggressive and controversial interrogation plan for one detainee.

The memos, 54 pieces altogether, were released by the American Civil Liberties Union (ACLU), a New York-based human rights group.

As the memos showed, FBI agents on temporary assignment at Guantanamo brought their concerns to the prison's commander, Maj. Gen. Geoffrey Miller, and laid them out in detailed messages to top officials at FBI headquarters in Washington.

One memo dated in May 2003 described tensions between the FBI agents and their military counterparts at Guantanamo over "aggressive interrogation tactics which are of questionable effectiveness and subject to uncertain interpretation based on law and regulation."

A military investigation into FBI reports of prisoner abuse at Guantanamo recommended that Miller be reprimanded for failing to oversee the interrogation of a high-value detainee, which was found to have been abusive.

But a top U.S. general rejected the recommendation and Miller recently requested early retirement.

Some 490 detainees are being jailed at Guantanamo, most of whom were captured in the U.S.-led war in Afghanistan in 2001 and were being held indefinitely without a trial.

Last year, an internal investigation of the U.S. military found many incidents of prisoner abuse there.

Source: Xinhua