US Senator Carl Levin said on Wednesday that their investigation, which looked into FBI allegations of abuse of prisoners, said abuse was not the result of a minority element.
"It is clear from the report that detainee mistreatment was not simply the product of a few rogue military police in a night shift," said Levin, a Michigan Democrat.
Bush administration officials have sought to portray the excesses at Abu Ghraib prison in Iraq as just that.
But looking into FBI reports of abuse, the investigators found multiple instances at the prison, including the use of duct tape on at least one prisoner's face, a threat to kill another prisoner's family, and inappropriate touching by female interrogators.
Investigators also determined that interrogators violated the Geneva Conventions and Army regulations three times. It was unclear from the aide's description what those instances were.
Previous investigations of prisoner abuse in Afghanistan, Iraq and Guantanamo have hurt US standing worldwide, which have prompted calls for a more thorough report.
The latest military investigation was conducted by Air Force Lieutenant General Randall Schmidt and Army Brigadier General John Furlow after the FBI agents' reports of abuse at Guantanamo surfaced last year.
They recommended that Army Major General Geoffrey Miller be reprimanded for failing to oversee the interrogation of a high-value detainee, which was found to have been abusive, the aide said.
Guantanamo holds 520 prisoners
No officer of Miller's rank or higher has been officially admonished in connection with any of the abuse scandals.
Former Brigadier General Janice Karpinski, who was in charge of Abu Ghraib prison in Iraq, is the highest-ranking officer to face punishment, despite calls from human rights groups to hold more senior leaders accountable.
Miller, a subject of criticism by human rights groups, took command of the prison camp at Guantanamo in late 2002 with a mandate to get more and better information from prisoners.
He later went to Iraq to oversee detainee operations there. He is now stationed at the Pentagon in a position unrelated to prisoners.
Guantanamo holds 520 prisoners, while more than 230 others have been released or transferred to the custody of their home governments.
Most were captured during the US invasion of Afghanistan after the 9/11 attacks; only a few have been charged with any crime.
One prisoner is Muhammad al-Qahtani, an alleged "20th hijacker" in the 9/11 plot, who military officials have acknowledged has been subject to extremely harsh interrogation techniques.
There have also been repeated accusations that American personnel at Guantanamo mishandled the Quran, the Muslim holy book. A separate Pentagon investigation found five such instances.