The Taliban is claiming responsibility for an attack on a base in eastern Afghanistan that left eight Americans and one Afghan dead.
Wednesday's suicide bombing resulted in the worst loss of life for the U.S. in the war torn nation since October.
A senior U.S. State Department official said the victims were not soldiers, but CIA workers and government contractors.
A suicide bomber was able to get past security at a remote U.S. outpost in Khost province by wearing a military uniform. The bomber concealed the explosives in a vest underneath his clothes, which he detonated once he got inside the gym.
"I am working at a construction company and guarding the same office at night. This company is close to the American airfield," Akbar Khan, a witness to Thursday's tragedy said.
He recalled, "I heard an explosion but the explosion looked inside the base and we do not know what happened inside."
Hundreds of American civilians, including intelligence agents, work in the Taliban stronghold.