The bomber had been recruited by the Jordanian intelligence service and taken to Afghanistan to infiltrate al-Qa'ida by posing as a foreign jihadi, US officials said.
The attacker, a physician-turned-mole, had been recruited to infiltrate al-Qa'ida's senior circles and had gained the trust of his CIA and Jordanian handlers with a stream of useful intelligence leads, two former senior officials briefed on the agency's internal investigation told The Washington Post. His track record as an informant apparently allowed him to enter a key CIA post without a thorough search, the sources said.
An Afghan security official identified the bomber as Hammam Khalil Abu Mallal al-Balawi, also known as Abu Dujana al-Khurasani. The Pakistani Taliban also claimed Balawi was the bomber, Arabic-language websites reported.
The bomber appears to have been invited to an operational planning meeting on al-Qa'ida, a former senior US intelligence official said. "It looks like an al-Qa'ida double-agent," the former official said. "It's very sophisticated for a terrorist group that's supposedly on the run."
The blast on December 30 killed four CIA officers, including the Khost base chief; three CIA contractors; and Mr bin Zeid, officials said. Six CIA employees were wounded in the attack.
The Al Jazeera television network reported the bomber had initially been recruited to provide intelligence on the whereabouts of bin Laden's top deputy, Ayman al Zawahiri.
The CIA's deputy chief of station from Kabul travelled to the meeting at the CIA Khost base, Forward Operating Base Chapman, according to former intelligence officials, pointing to the meeting's importance. The officer was wounded in the attack, according to informed sources.
Both the Pakistani Taliban and the Afghan Taliban have claimed responsibility for the attack. The Afghan Taliban fights alongside an array of militants, including the Haqqani network, an Islamic extremist group that operates in Afghanistan and Pakistan and maintains close ties to al-Qa'ida.
The US and Jordanian intelligence services have worked closely together for years, said a former senior intelligence official. "There's a confidence level with them," the former official said.
Officials said Balawi had been jointly managed by US and Jordanian agencies and had provided "actionable intelligence" over several weeks of undercover work along the Afghanistan-Pakistan border.
"This is someone they obviously trusted very, very much," a former official said.
Balawi was an active recruiter and an "elite writer" on al-Qa'ida's password-protected al-Hisba website, where he went by the name Abu Dujana al-Khurasani, according to the journal of the US Military Academy at West Point's Combating Terrorism Centre.
In a posting on the site in May 2007, Balawi sought to persuade people from a variety of backgrounds, including African-Americans, Native Americans, Vietnamese and poor immigrants, to join the fight against their "oppressor", the US, the West Point analysts found.
Balawi had studied medicine in Turkey with government funding, according to a translation of the Jordanian website Jerasa News by the Middle East Media Research Institute. He left Jordan about a year ago after being detained for a few months by Jordanian intelligence officers.