WASHINGTON - In northern Bucharest, in a busy residential neighborhood minutes from the heart of the capital city, is a secret the Romanian government has long tried to protect.

For years, the CIA used a government building  codenamed "Bright Light"  as a makeshift prison for its most valuable detainees. There it held al Qaeda operatives Khalid Sheikh Mohammed, the mastermind of 9/11, and others in a basement prison before they were ultimately transferred to Guantanamo Bay, Cuba, in 2006, according to former U.S. intelligence officials familiar with the location and inner workings of the prison.

The existence of a CIA prison in Romania has been widely reported, but its location has never been made public. The Associated Press and German public television ARD located the former prison and learned details of the facility where harsh interrogation tactics were used. ARD's program on the CIA prison is set to air Thursday.

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The Romanian prison was part of a network of so-called black sites that the CIA operated and controlled overseas in Thailand, Lithuania and Poland. All the prisons were closed by May 2006, and the CIA's detention and interrogation program ended in 2009.

Unlike the CIA's facility in Lithuania's countryside or the one hidden in a Polish military installation, the CIA's prison in Romania was not in a remote location. It was hidden in plain sight, a couple blocks off a major boulevard on a street lined with trees and homes, along busy train tracks.

The building is used as the National Registry Office for Classified Information, which is also known as ORNISS. Classified information from NATO and the European Union is stored there. Former intelligence officials both described the location of the prison and identified pictures of the building.

In an interview at the building in November, senior ORNISS official Adrian Camarasan said the basement is one of the most secure rooms in all of Romania. But he said Americans never ran a prison there.

"No, no. Impossible, impossible," he said in an ARD interview for its "Panorama" news broadcast, as a security official monitored the interview.

The CIA prison opened for business in the fall of 2003, after the CIA decided to empty the black site in Poland, according to former U.S. officials, who spoke on condition of anonymity because they were not authorized to discuss the detention program with reporters.

Shuttling detainees into the facility without being seen was relatively easy. After flying into Bucharest, the detainees were brought to the site in vans. CIA operatives then drove down a side road and entered the compound through a rear gate that led to the actual prison.

The detainees could then be unloaded and whisked into the ground floor of the prison and into the basement.

The basement consisted of six prefabricated cells, each with a clock and arrow pointing to Mecca, the officials said. The cells were on springs, keeping them slightly off balance and causing disorientation among some detainees.

The CIA declined to comment on the prison.

During the first month of their detention, the detainees endured sleep deprivation and were doused with water, slapped or forced to stand in painful positions, several former officials said. Waterboarding, the notorious interrogation technique that simulates drowning, was not performed in Romania, they said.

After the initial interrogations, the detainees were treated with care, the officials said. The prisoners received regular dental and medical checkups. The CIA shipped in Halal food to the site from Frankfurt, Germany, the agency's European center for operations. Halal meat is prepared under religious rules similar to kosher food.

Former U.S. officials said that because the building was a government installation, it provided excellent cover. The prison didn't need heavy security because area residents knew it was owned by the government. People wouldn't be inclined to snoop in post-communist Romania, with its extensive security apparatus known for spying on the country's own citizens.