(CBS/AP) Last updated at 6:47 p.m. EST
The suicide bomber who killed seven CIA employees and wounded six more at a remote outpost in southeastern Afghanistan had been invited onto the base and had not been searched, two former U.S. officials have told The Associated Press.
A former senior intelligence official says the man was being courted as an informant and that it was the first time he had been brought inside the camp.
The official says a senior and experienced CIA debriefer came from Kabul for the meeting, suggesting that the purpose of the session was to gain intelligence.
The former intelligence officials and another former official with knowledge of the attack spoke on condition of anonymity because they were not authorized to speak publicly.
The Central Intelligence Agency would not confirm the details, and said it was still gathering evidence.
"It's far too early to draw conclusions about something that happened just yesterday," said spokesman George Little.
A separate U.S. official suggested the bomber may have set off the explosives as he was about to be searched.
The bombing was expected to deal a major psychological blow to the spy agency, if not its ability to collect valuable intelligence on Taliban and al Qaeda forces operating along Afghanistan's eastern border with Pakistan. Officials credit the base with providing some of the intelligence which has enabled CIA drone strikes to eliminate much of al Qaeda's top leadership, reports CBS News correspondent David Marin.
The New York Times reports that the victims were responsible for collecting information about militant networks in Afghanistan and Pakistan, and plotting missions to kill the networks’ top leaders.
CBSNews.com Special Report: Afghanistan
Martin reports Afghan soldiers and civilians are present at almost every American outpost since one of the chief principals of the U.S. strategy is to partner with the Afghans. According to Christine Fair of Georgetown University, some of them may actually be working for the Taliban.
"They have really become a vehicle of infiltration for the Taliban," Fair said. "This is most certainly a vulnerability in our strategy going forward in trying to hand over security to the Afghans," said Fair; "If we don't really have a way of figuring out who we can trust."
CIA Director Leon Panetta said in a message to agency staff that the casualties sustained in Wednesday's strike at Forward Operating Base Chapman were the result of a terrorist attack.
Initial reports indicated that eight American civilians had been killed. There was no explanation for the discrepancy in Panetta's message, which was released by the CIA in an unusual step a day after one of the deadliest attacks on the Central Intelligence Agency in its history.
"Those who fell yesterday were far from home and close to the enemy, doing the hard work that must be done to protect our country from terrorism," Panetta said. "We owe them our deepest gratitude, and we pledge to them and their families that we will never cease fighting for the cause to which they dedicated their lives - a safer America."
"Yesterday's tragedy reminds us that the men and women of the CIA put their lives at risk every day to protect this nation," he said. "Throughout our history, the reality is that those who make a real difference often face real danger."
No further information about the victims would be released," the CIA director said, "due to the sensitivity of their mission and other ongoing operations."
President Barack Obama said the killed CIA employees come from a "long line of patriots" whose courageous service has helped to thwart terrorist plots and save lives.
In a letter to CIA employees released by the agency, Mr. Obama said the U.S. would not be able to maintain its freedom and security without their service. He also noted that the spy agency has been tested "as never before" since the Sept. 11 attacks.
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