In the historic city of Bucharest, the CIA tortured and interrogated high-risk terrorists for years in a basement prison, under the nose of roughly two million Romanians.

An investigation carried out by reporters for the Associated Press led to the news organization revealing on Thursday that in the years after the September 11 terror attacks, the Central Intelligence Agency operated an underground holding center for some of the most sought after alleged terrorists. There in a small six-cell jail, prisoners such as al-Qaeda operative Khalid Sheikh Mohammed were beaten and detained, all unbeknownst to the citizens of Romania or American authorities outside of the CIA.

This revelation from the AP comes after officials have adamantly denied any such institution in the past.

"No, no. Impossible, impossible," Adrian Camarasan of the National Registry Office for Classified Information told Germany’s ARD television in an earlier interview. While Camarasan dismissed claims of a top-secret jail, former intelligence officials speaking under condition of anonymity now tell the AP that the prison did in fact exist, and did so right in the basement of the National Registry’s headquarters.

The National Registry Office for Classified Information, also known as ORNISS, has operated in the Bucharest building for years. In its annals are classified files pertaining to NATO and European Union intelligence. A known government installation, townspeople avoided the structure and thus the CIA had the perfect cover to open the prison. The first detainees came arrived in 2006, and despite being mere blocks from a major roadway and active train tracks, the top-secret cells beneath the first floor of the ORNISS building went perfectly undetected — until now.

In those cells, insiders tell the AP, prisoners were interrogated once installed in the compound. They were subjected to sleep deprivation, doused with water and slapped by intelligence officers. Detainees were kept in small cells erected atop springs, as to disorient the prisoners as well.

Though the prison was kept open until 2006, not all that lasted through it were freed in the end. Some were returned to their home countries. Others, like Mohammed, were sent to Guantanamo Bay. In September of that year, then-President George W Bush said that Mohammed had been under the custody of the CIA for questioning but never revealed where that exactly was. The next year the al-Qaeda operative would confess from Gitmo his role as a 9/11 mastermind and has been detained in the elusive military prison under guard of the American armed forces ever since.

In their investigation of the prison, AP reporters were able to link CIA-chartered jets from Bucharest into other locales, including Guantanamo and other sites of known prisons.

Only those speaking anonymously to the AP have revealed the true nature and location of the Bucharest prison. When asked for comment, the CIA officially declined.

"There have been years of official denials," Dick Marty, a Swiss lawmaker who investigated claims of CIA secret prisons for the Council of Europe. "We are at last beginning to learn what really happened in Bucharest."