The CIA vowed on Thursday to avenge the deaths of seven officers in a suicide bombing in Afghanistan and to investigate security breaches that allowed the second deadliest attack in agency history.
The Taliban claimed the attacker was a sympathizer from the Afghan army who detonated a vest of explosives at a meeting with CIA workers on Wednesday. An Afghan was also killed and six CIA employees were wounded, U.S. officials said.
"This deadly attack was carried out by a valorous Afghan army member," Taliban spokesman Zabihullah Mujahid told Reuters.
In a letter to CIA employees, U.S. President Barack Obama mourned the deaths of those he said "served in the shadows." The death toll was the intelligence agency's highest since eight employees were killed in a bombing of the U.S. Embassy in Beirut in 1983.
The chief of the CIA base was among the dead, according to a former intelligence official. "We fully expected to lose agents, but to lose so many all at once is a huge shock to the system and is very troubling," he said.
The attack took place inside Forward Operating Base Chapman, a well fortified base in Khost province near the southeastern border with Pakistan, where the CIA has been stepping up operations to battle a resurgent Taliban.
The bombing highlighted the insurgency's reach and coordination at a time when violence has reached its highest levels since the overthrow of the Taliban regime by U.S.-backed Afghan forces in 2001.
CIA Director Leon Panetta said the deaths would not deter the agency. "This attack will be avenged through successful, aggressive counterterrorism operations," a U.S. intelligence official said on condition of anonymity.
U.S. State Department spokeswoman Julie Reside said plans to increase the U.S. civilian presence in Afghanistan remained on track and that security would continue to be a primary concern.
Also on Wednesday, five Canadians -- four soldiers and a journalist -- were killed when their armored vehicle was hit by a bomb in southern Kandahar province, the Canadian Defense Ministry said.
Trusting Afghan forces
The Afghan Defense Ministry rejected the Taliban's claim that an Afghan soldier was involved in the attack and said none were stationed at the base. But a spokesman for NATO-led forces in Afghanistan acknowledged Afghan security forces were working there.
If the bomber does prove to be from the army, it would mark the second deadly attack in three days on foreign troops and officials by Afghan soldiers being groomed to eventually take over the nation's security.
Obama has started deploying 30,000 extra troops to tackle the violence and NATO allies are contributing thousands more.
The surge is scheduled to be scaled back starting in 2011 as the United States gradually hands security province by province over to the Afghans.
Security lapses at U.S. bases have been in the spotlight since a U.S. army psychiatrist allegedly killed 13 people in a November 5 shooting spree at the Fort Hood army base in Texas.
One U.S. official, speaking on condition he not be named, pointed to the Fort Hood incident as evidence that problems spotting potentially dangerous personnel were not limited to Afghanistan.
"Any time you have an incident like this, it gives you an opportunity to evaluate a whole range of things, whether it's vetting procedures, whether it's security procedures, whether it's intel, whether it's physical security," he said.
The CIA did not say how long its investigation would take.
"There's still a lot to be learned about what happened. The key lesson is that counterterrorism work is dangerous," a CIA spokesman said.
The blast that killed the five Canadians struck the patrol as it was visiting reconstruction projects near Kandahar.
The journalist killed, Michelle Lang, 34, was on her first assignment in Afghanistan. She is the third journalist to die in Afghanistan this year.