The unit that operated the detention center reported directly to the office of Prime Minister Nuri Al-Maliki but officials denied any connection to or knowledge of the facility in Al-Maliki's inner circle.

"The three officers were detained for interrogation," Kamil Ameen, a spokesman for Iraq's human rights ministry, told Reuters. "What matters to us is that the prison has been closed and it is now part of the past." The closure of the secret jail comes at a sensitive time for Al-Maliki, who is fighting to hold onto his position as prime minister following an inconclusive general election in March.

A cross-sectarian alliance backed by Iraq's Sunni Arab minority won the largest number of seats, though Al-Maliki's bloc came a close second. Neither has enough seats to form a majority government.

The prison, located at the Muthanna airfield in central Baghdad, held suspected insurgents captured during a military campaign in the northern province of Nineveh last year.

The prisoners should have been handed over to the Justice Ministry for trial but were not.

The Human Rights Ministry said it sent investigators to the facility when it found out about it a few weeks ago and that the detainees there complained they were abused and not allowed to contact their families or seek legal assistance. According to the Los Angeles Times, some prisoners say they were raped.

"There were 431 detainees, 100 of them were released, 20 of them were sent back to Mosul... and the rest were sent to Justice Ministry prisons," Ameen said, referring to the main city in Nineveh.

Military officials said they had warrants for the arrest of all the detainees held at the facility.

"It is not a secret prison, but is a detention cell which is open to all human rights organizations and the Human Rights Ministry," Baghdad's security spokesman, Maj. Gen. Qassim Al-Moussawi, told Reuters earlier this week.

"I deny the existence of any secret prison under the authority of the (government) or... of any security force linked to the prime minister." Human Rights Minister Wijdan Salim said in a recent televised discussion that he did not believe there were any other extrajudicial detention centers in Iraq.

A US embassy spokesman said the embassy had raised concerns with senior Iraqi officials over reports of the extrajudicial detention facility and allegations of abuse.

Seven years after Saddam's ouster, about 100,000 US troops remain in Iraq. The US military plans to cut that number to 50,000 by the end of August and for all troops withdraw by the end of 2011.