KABUL, Afghanistan — A suicide bomber wearing an explosive vest killed at least eight American civilians, most of them C.I.A. officers, at a remote base in southeastern Afghanistan on Wednesday, according to NATO officials and former American intelligence officials.
The attack at the C.I.A. base, Forward Operating Base Chapman, in Khost Province appeared to be the single deadliest episode for the spy agency in the eight years since the Sept. 11, 2001, attacks. It also dealt a significant blow to the often insular, tight-knit organization, which has lost only 90 officers in the line of duty since its founding in 1947.
One former C.I.A. official said that eight agency employees had been killed but cautioned that early reports from the field were often incorrect.
The official, who spoke anonymously because the agency had not commented publicly on the attack, said the final number of dead could be higher because at least six American civilians were wounded. It was unclear how many of the dead were full-time C.I.A. officers and how many were contract employees.
A C.I.A. spokesman did not return calls seeking comment. Previously, the spy agency has revealed that four of its officers have been killed since the Sept. 11 attacks, all of them memorialized with stars carved into a marble wall at C.I.A. headquarters in Langley, Va.
In recent years, the C.I.A. has been at the forefront of American counterterrorism operations in South Asia, launching a steady barrage of drone attacks against Qaeda and Taliban operatives in the mountains along the Afghanistan-Pakistan border.
There was a large Qaeda camp south of Khost until August 1998, when its inhabitants were forced to move into Pakistan after the United States, using cruise missiles, bombed the camp in response to the attacks on American embassies in East Africa, believed to have been planned by Al Qaeda.
A NATO official said the bomber managed to elude security and reach an area near the base’s gym. It was not clear whether the bomber, who apparently died in the blast, entered the gym. Among other questions raised was whether the bomber worked at the base and had clearance, or if a security slip allowed him to gain entry, and whether other operatives had access to the base.
The former C.I.A. official said the agency was in the process Wednesday of notifying family members of the officers killed in the attack. The names of some of those killed might never be publicly disclosed, because several of the C.I.A. officers may have been working undercover.
American bases in Khost, particularly Camp Salerno, one of the largest in the country, have been the targets of frequent attacks. Dozens of Afghan laborers have been killed in bombings over the past several years.
In May, a suicide attacker exploded a car bomb near the main gate, killing 7 civilians and wounding 21. The bombing took place a day after a coordinated Taliban attack inside the city of Khost, the provincial capital, that left at least 7 civilians and 8 insurgents dead.
In June, a man riding a motorcycle detonated explosives near a densely crowded intersection in the city, killing 7 Afghan civilians and wounding 44, including 7 children.
In recent weeks, American officials have stepped up pressure on Pakistan to root out the Haqqani network, whose fighters pose one of the greatest threats to American forces and hold sway over large parts of Afghanistan, including Paktika, Paktia and Khost Provinces.