One thing that troubled me from the first when I heard about the attack on our CIA base in Afghanistan was how did the attacker know it was a focal point for our intel efforts?

It turns out that the attacker knew because he was a CIA informant who came and went with relative ease at the base. Aleem Agha and Nick Schifrin of ABC News has the story:

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 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. 

It has also been learned that the suicide bomber was driven across the border from Pakistan into Afghanistan which raises the ugly possibility that Pakistani intelligence may have been aware of the plot.

This appears to have been a telling blow to our counterterror capabilities in Afghanistan and will no doubt take several months to reconstitute.