WASHINGTON // A few hundred metres from the administrative offices of the Guantanamo Bay prison, hidden behind a ridge covered in thick scrub and cactus, sits a closely held secret.

A dirt road winds its way to a clearing where eight small cottages sit in two rows of four. They have long been abandoned. The special detachment of marines that once provided security is gone.

But for several years after the 9/11 attacks in New York, these cottages were part of a covert CIA programme. Its secrecy has outlasted black prisons, waterboarding and rendition.

In these buildings, CIA officers turned terrorists into double agents and sent them home.

It was a risky gamble. If it worked, their agents might help the CIA find terrorist leaders to kill with drones. But officials knew there was a chance that some prisoners might quickly spurn their deal and kill Americans.

For the CIA, that was an acceptable risk in a dangerous business. For the American public, which was never told, it was one of the many secret trade-offs the government made on its behalf.

At the same time the government used the threat of terrorism to justify imprisoning people indefinitely, it was releasing dangerous people from prison to work for the CIA.

Nearly a dozen current and former US officials have described aspects of the programme. All spoke on condition of anonymity because they were not authorised to publicly discuss the secret programme, even though it ended in about 2006.

The programme and the handful of men who passed through these cottages had various official CIA code names. But those who were aware of the cluster of cottages knew it best by its sobriquet: Penny Lane.

It was a nod to the classic Beatles song and a riff on the CIA’s other secret facility at Guantanamo Bay, a prison known as Strawberry Fields.

Some of the men who passed through Penny Lane helped the CIA find and kill many top Al Qaeda operatives, said US officials. Others stopped providing useful information and the CIA lost touch with them.

When prisoners began streaming into the prison on the US naval base at Guantanamo Bay, Cuba, in January 2002, the CIA recognised it as an unprecedented opportunity to identify sources. That year, 632 detainees arrived at the island. The following year 117 more arrived.

By early 2003, Penny Lane was open for business.

Candidates were ushered from the confines of prison to Penny Lane’s relative comfort, officials said. The cottages had private kitchens, showers and televisions. Each had a small patio.

Some prisoners asked for and received pornography. One official said the biggest luxury in each cottage was the bed, not a military-issued cot but a real bed with a mattress.

The officials said dozens of prisoners were evaluated but only a handful, from varying countries, were turned into spies who signed agreements to spy for the CIA.

The US government said it confirmed that about 16 per cent of former Guantanamo Bay detainees rejoined the fight against America.

Infiltrating Al Qaeda has been one of the CIA’s most sought-after but difficult goals, something that other foreign intelligence services have only occasionally accomplished. So candidates for Penny Lane needed legitimate terrorist connections. To be valuable to the CIA, the men had to be able to reconnect with Al Qaeda.

From what the administration of the former US president, George W Bush, said about Guantanamo Bay prisoners at the time, the CIA would have seemingly had a large pool to draw from.

In reality, many were held on flimsy evidence and were of little use to the CIA.

While the agency looked for viable candidates, those with no terrorism ties sat in limbo. It would take years before the majority of detainees were set free, having never been charged.

Of the 779 people who were taken to Guantanamo Bay, more than three quarters have been released and that was mostly during the Bush administration.

Prisoners agreed to cooperate for a variety of reasons, officials said. Some received assurances that the US would resettle their families. Another thought Al Qaeda had perverted Islam and believed it was his duty as a Muslim to help the CIA destroy it.

All were promised money. Exactly how much each was paid remains unclear. But altogether, the government paid millions of dollars for their services, officials said.

The money came from a secret CIA account, code-named Pledge, that was used to pay informants, officials said.

The biggest fear, the officials involved with the programme recalled, was that a former detainee would attack Americans, then publicly announce that he had been on the CIA payroll.

Al Qaeda suspected the CIA would have attempted such a programme and its operatives have been suspicious of former Guantanamo Bay detainees, intelligence officials and experts said.

* Associated Press