Bulgaria is mentioned in a report that Amnesty International released, exposing a covert operation whereby people have been arrested and transferred to countries where they have faced torture and other ill-treatment.

The report describes how the CIA has used private aircraft operators and front companies to preserve the secrecy of "rendition" flights. Rendition is the illegal transfer of people from one country to another in ways that bypass all judicial and administrative oversight.

The new document shows that the CIA has used covertly aviation practices that would otherwise require their flights to be declared to aviation authorities.

Based on reports from prisoners, Amnesty concluded that they were likely held in at least three different countries.

The prisoners were provided Internet printouts of prayer schedules for their location. Because Muslim prayer times are based on sunrise and sunset, it was possible to determine that the location was above the 41st parallel -- a position that coincides with a site in Turkey, Azerbaijan, Georgia, Romania, Bulgaria, Albania or Macedonia.

Amnesty International has records of nearly 1,000 flights directly linked to the CIA, most of which have used European airspace.

Amnesty also lists private airlines with permission to land at US military bases worldwide.

Amnesty International cautioned that states that tolerate these flights landing on their territory and companies that carry them out, may find themselves complicit in serious human rights abuses.