The CIA has been hiding and interrogating some of its most important al-Qaida captives at a Soviet-era compound in Eastern Europe, according to US and foreign officials familiar with the arrangement, the Washington Post reported. The secret facility is part of a covert prison system set up by the CIA nearly four years ago that at various times has included sites in eight countries, including Thailand, Afghanistan and several democracies in Eastern Europe, as well as a small center at the Guantanamo Bay prison in Cuba, according to current and former intelligence officials and diplomats from three continents, the paper said Tuesday on its Web site. The hidden global internment network is a central element in the CIA's unconventional war on terrorism, the Post said. It depends on the cooperation of foreign intelligence services, and on keeping even basic information about the system secret from the public, foreign officials and nearly all members of Congress charged with overseeing the CIA's covert actions. The existence and locations of the facilities, referred to as "black sites" in classified White House, CIA, Justice Department and congressional documents, are known to only a handful of officials in the United States and, usually, only to the president and a few top intelligence officers in each host country, it said.