The CIA sent some Guantanamo Bay detainees back home as double agents and used their information to help find and kill terrorists, the Associated Press reports. Anonymous US officials tell the AP that prisoners were offered millions of dollars, freedom, and safety for their families in exchange for help. Though only a small number of prisoners are said to have been part of the program, the AP reports that they collectively helped the CIA find and kill many top members of al-Qaeda. The program reportedly began after 9/11 and was ended in 2006.

According to the AP, some aspects of the program were carried out from a secret location around 300 yards from the prison's administrative offices, in a series of facilities hidden behind a ridge. The secret site held eight cottages built to feel like hotel rooms rather than prison cells. It was given the nickname Penny Lane, playing off of another secret Guantanamo Bay facility known as Strawberry Fields — both the names of Beatles songs that appeared on a single together. It's unclear exactly what activities occurred at Penny Lane, and what occurred as a result of sending prisoners back overseas.

Though the CIA was reportedly concerned that some prisoners would abandon them and begin targeting the US again, officials that spoke with the AP said that they knew of no instances where one later killed an American. However, others who were kept at Guantanamo Bay but were not part of the program did turn back to fighting the US: reportedly around 16 percent of released detainees are confirmed to have done so, and officials suspect — without confirmation — that another 12 percent did so as well.

All prisoners who joined the secret program were promised money, though the AP says that they had various reasons for agreeing to take part. At least one person only agreed to the plan after the CIA implied that it might harm his children. And while some of the double agents are said to have helped CIA missions, the AP reports that others eventually stopped contacting the agency or ceased to give it any useful information. Though the program is said to have ended in 2006, none of the officials the AP spoke with were authorized to speak about the program yet, and its exact name is still unknown.