The attack represented an audacious blow to intelligence operatives at the vanguard of U.S. counterterrorism operations in both Afghanistan and Pakistan, killing officials whose job involves plotting strikes against the Taliban, al-Qaeda and other extremist groups that are active on the frontier between the two nations. The facility that was targeted -- Forward Operating Base Chapman -- is in the eastern Afghan province of Khost, which borders North Waziristan, the Pakistani tribal area that is believed to be al-Qaeda's home base.
U.S. sources confirmed that all the dead and injured were civilians and said they believed that most, if not all, were CIA employees or contractors. At least one Afghan civilian also was killed, the sources said.
It is unclear exactly how the assailant managed to gain access to the heavily guarded U.S.-run post, which serves as an operations and surveillance center for the CIA. The bomber struck in what one U.S. official described as the base's fitness center.
In addition to the dead, eight people were wounded, several of them seriously, U.S. government officials said.
While many details remained vague Wednesday, the attack appears to have killed more U.S. intelligence personnel than have died in Afghanistan since the U.S.-led invasion began in late 2001. The CIA has previously acknowledged the deaths of four officers in fighting in Afghanistan in the past eight years.