The suicide bomber who killed at least six Central Intelligence Agency officers in a base along the Afghan-Pakistan border on Wednesday was a regular CIA informant who had visited the same base multiple times in the past, according to someone close to the base's security director.
The informant was a Pakistani and a member of the Wazir tribe from the Pakistani tribal area North Waziristan, according to the same source. The base security director, an Afghan named Arghawan, would pick up the informant at the Ghulam Khan border crossing and drive him about two hours into Forward Operating Base Chapman, from where the CIA operates.
Because he was with Arghawan, the informant was not searched, the source says. Arghawan also died in the attack.
The story seems to corroborate a claim by the Taliban on the Pakistani side of the border that they had turned a CIA asset into a double agent and sent him to kill the officers in the base, located in the eastern Afghan province of Khost.
The infiltration into the heart of the CIA's operation in eastern Afghanistan deals a strong blow to the agency's ability to fight Taliban and al Qaeda, former intelligence officials say, and will make the agency reconsider how it recruits Pakistani and Afghan informants.
The officers who were killed in the attack were at the heart of the United States' effort against senior members of al Qaeda and the Taliban, former intelligence officials say. They collected intelligence on the militant commanders living on both sides of the border and helped run paramilitary campaigns that tired to kill those commanders, including the drone program that has killed a dozen senior al Qaeda with missiles fired from unpiloted aircraft.
The former intelligence officials all say the CIA will be able to replace those who were killed, but the officials acknowledge the attack killed decades of knowledge held by some of the agency's most informed experts on the region, the Taliban and al Qaeda. It also killed at least one officer who had been part of the agency's initial hunt for Osama bin Laden in the mid-1990s.
Suicide Attack Likely Revenge for Drone Attacks on Taliban Leaders
"This is a tremendous loss for the agency," says Michael Scheuer, a former CIA analyst who led the bin Laden unit. "The agency is a relatively small organization, and its expertise in al Qaeda is even a smaller subset of that overall group."
At least 13 officers gathered in the base's gym to talk with the informant, suggesting he was highly valued. His prior visits to the base and his ability to get so close to so many officers also suggests that he had already provided the agency with valuable intelligence that had proven successful, former intelligence officials say.
That information was most likely linked with the CIA's drone program on the Pakistani side of the border. The Taliban in Pakistan claimed in a call to The Associated Press that the informant had called them and offered to become a turncoat. They said the attack was revenge for the drone attacks that have killed multiple senior Taliban leaders.
"The war that is going on between the CIA and the bad guys in this part of the world is a real war," says Tim Weiner, author of "Legacy of Ashes: The History of the CIA." "The CIA is targeting them with missiles fired by drone aircraft and they are taking their revenge in the way they know best -- which is to kill the people who are trying to kill them. ... This attack is one of the consequences of using targeted killing as a weapon of war."
The drone program relies on informants who can cross the border easily, in parts CIA officers cannot. The CIA is wary of making major trips into Pakistan for fear of the repercussions if officers were caught or killed there. Informants -- especially Pakistani informants from the Waziri or Mehsud tribe -- are the most valuable assets for the CIA in finding senior al Qaeda and Taliban militants who are targeted by the drone program.
"To go after the Taliban and the Haqqani network on the Pakistani side of the border, the United States relies almost exclusively on its predator drones. But those predator drones require agents on the ground to direct them, to say, 'this is where you should be looking,'" says Richard Clarke, the Bush administration's counter-terrorism czar until 2003 and an ABC News consultant. "The CIA does that in support of the military, and without their intelligence, we really have very little way of affecting what's going on on the Pakistani side of the border."
New Methods to Recruit Informants in Afghanistan
The most likely Taliban group to have perpetrated the attack is the one led by Sirajuddin Haqqani, the son of Jalaluddin Haqqani, one of the CIA's most important assets when the agency was helping fund the Afghan mujahedeen fighting the Soviets in Afghanistan. The Haqqanis have been running militant operations for 30 years and have recently become perhaps the most lethal commanders targeting U.S. troops in Afghanistan. They are based in North Waziristan but control large parts of Khost and other provinces in eastern Afghanistan as well.
The Haqqanis have also kidnapped the only known American soldier in enemy custody -- PFC Bowe Bergdahl -- according to a senior NATO official. Since Bergdahl was kidnapped in late June, the official says the Haqqanis "have been getting pounded" and a "great many of their mid to senior leaders have been captured and/or killed."
The infiltration into the CIA base suggests an extremely high level of sophistication, even for a network that has a huge reach across the area.
"The Soviet Union during the Cold War, the Cubans during the Cold War were able to run double agents against the CIA very successfully," says Clarke. "But for a non-nation state to be able to do this -- for the Haqqani network of the Taliban to be able to do this -- represents a huge increase in the sophistication of the enemy."
Clarke and other former intelligence officials predict the CIA in Afghanistan will be forced to question who they can trust and change their methods in how they find informants.
"Because of this attack, the CIA will be very, very careful about who it trusts, how it vets its agents, how it searches its agents," Clarke says. "And this will mean that in the future, it will be much more difficult for CIA to operate in the field because of probably new security roles affecting what they do."
More Victims in Suicide Bomber Attack Identified
The only victim of the attack who has been publicly identified is 37-year-old Harold Brown Jr., a father of three. The base chief, a woman in her 30s, was also killed, according to current and former intelligence officials. She is believed to have been focused on al Qaeda since before 9/11. A former U.S. official says a second woman was also killed in the attack, and that both women had "considerable counterintelligence experience."
The attack also killed Captain Al Shareef Ali bin Zeid, a member of the Jordanian spy agency Dairat al-Mukhabarat al-Ammah, according to people who have spoken with bin Zeid's family. The Jordanian military released a statement acknowledging bin Zeid had been killed in Afghanistan, but did not mention he was working with the CIA.
The ability for the CIA to replace the officers and institutional knowledge taken away by the attack will help decide the war in Afghanistan, former intelligence officials argue.
CIA historian Tim Wiener says, "The war we are in is going to be won or lost by the quality of intelligence the CIA can gather in the field."
ABC News' Kirit Radia and Nadine Shubailat contributed to this report.