The hitherto secret location of what is said to have been one of the CIA's notorious interrogation prisons in Eastern Europe has been traced to a government building in the Romanian capital, Bucharest.

The prison, code-named "Bright Light", was allegedly housed until 2006 in the basement of Romania's National Registry Office for Classified Information, which lies in a busy residential district. Detainees, who included al-Qa'ida figures who masterminded the 9/11 attacks, were held in cells set on springs to keep them off balance, repeatedly slapped, doused with water and forced to stand for hours in painful positions.

The prison – which belonged to a string of so-called CIA "black sites" in Thailand, Lithuania, and Poland – was discovered during a joint investigation by the Associated Press and the German television channel ARD, which aired a documentary on the subject last night. The programme-makers said the CIA selected a government building in what had been one of the East Bloc's infamous police states to minimise the possibility of being detected.

The existence of a CIA prison in Romania has been widely reported, but although the CIA ended its detention and interrogation programme in 2009, its whereabouts had never been established.

Romania has denied providing prison facilities for the CIA and government officials interviewed in the documentary claim that the idea is "impossible". But the film-makers were tipped off about the prison by former US intelligence officials.

Prisoners were flown into Bucharest, picked up from the airport in windowless vans and entered the tree-lined compound via a rear gate. Inside, the detainees were held in six prefabricated cells. Officials insisted the technique of waterboarding was never used at the facility and that after the initial harsh interrogation the prisoners were provided with full medical treatment and special food to meet the requirements of their religion. The prisoners were later transferred to Guantanamo Bay.