A suicide bomber who killed seven CIA agents at a meeting at a remote base in eastern Afghanistan in January had not been properly vetted, the CIA has said.
Humam Khalil Abu-Mulal al-Balawi, a Jordanian al-Qaeda sympathiser, blew himself up after promising to give crucial intelligence on top US targets.
In a letter to employees, CIA director Leon Panetta cited a range of failures that gave him proximity to the agents.
These included a CIA officer failing to pass on warnings about the bomber.
The attack was the worst against US intelligence officials since 1983.
Mr Panetta said a classified internal inquiry into the incident had found al-Balawi had already provided intelligence that had been independently corroborated, and CIA officers believed he had more to offer.
"He had confirmed access within extremist circles, making a covert relationship with him - if he was acting in good faith - potentially very productive," Mr Panetta wrote. "But he had not rejected his terrorist roots."
Mr Panetta said the internal report identified shortcomings across several different parts of the US spy agency. He said warnings about the double agent were not shared, and that security measures at Forward Operating Base Chapman were insufficient.
The BBC's Steve Kingstone, in Washington, says one of the most disturbing findings is that the CIA was warned by Jordanian intelligence that its prized informant might, in fact, be a double-agent working for al-Qaeda.
That was almost a month before the suicide bombing, but the CIA officer in Jordan failed to pass the warning on.
"Responsibility cannot be assigned to any particular individual or group," Mr Panetta told employees. Mr Panetta said that despite the fatal errors, no intelligence officials would be disciplined.
'Determination influenced judgements'
He added that zeal to disrupt al-Qaeda had contributed to al-Balawi's being let into the base.
"It was the intense determination to accomplish the mission that influenced the judgements that were made," Mr Panetta said.
Al-Balawi, 36, had been a doctor in Jordan, and had been arrested by Jordanian intelligence a year before the deadly attack at Forward Operating Base Chapman.
He was then reportedly recruited by the Jordanians and the CIA, who wrongly thought they had turned him and that he had reformed, and given a mission to find al-Qaeda leaders.
Al-Balawi had even fooled his relatives about his intentions and his beliefs, telling his family he was travelling to Turkey to join his Turkish wife and children and to continue his medical studies.
Shortly after the attack, al-Jazeera television aired a video purporting to show al-Balawi with Pakistani Taliban leader Hakimullah Mehsud, vowing revenge on the US for the killing of a previous Taliban leader.
The 1983 bombing of the CIA station in Beirut killed 17 officers.