Kabul, Afghanistan (CNN) -- Two of the seven CIA officers killed Wednesday in a suspected terrorist attack on a U.S. base in Afghanistan were contractors for Xe, a private security firm formerly known as Blackwater, a former intelligence official said Thursday.

A current intelligence official confirmed to CNN that the casualties included a mix of people -- CIA staff and contractors. The CIA considers contractors to be officers.

The Taliban have claimed responsibility for Wednesday's attack, which wounded six other people. A senior U.S. official said information suggested that a bomber walked into a gym facility at Forward Operating Base Chapman -- in Khost Province, near the border of Pakistan -- and detonated bombs in a suicide vest.

It is not known how the bomber got past security. In a posting on its Web site Thursday, the Taliban claimed the bomber was an Afghan National Army soldier.

A U.S. intelligence official on Thursday vowed that the United States would avenge the attack.

"This attack will be avenged through successful, aggressive counterterrorism operations," the intelligence official vowed.

Former CIA official Robert Richer called it "the greatest loss of life for the Central Intelligence Agency since the Beirut Embassy bombing" in 1983, which killed eight agents.

Flags at CIA headquarters in Langley, Virginia, flew at half-staff Thursday.

"These brave Americans were part of a long line of patriots who have made great sacrifices for their fellow citizens, and for our way of life," President Obama said in a written statement Thursday.

"The United States would not be able to maintain the freedom and security that we cherish without decades of service from the dedicated men and women of the CIA."

Richer, who retired from the agency in 2005 as the associate deputy director for operations, knew many of Wednesday's victims personally. In a written statement Thursday, he called on the public to "remain mindful that our great country is served well by those in and out of uniform. That officers like those lost in this attack placed themselves in harm's way, at a critical and dangerous crossroads in the war on terror.

"We should be thankful for the service and sacrifice of these fine Americans. They and their colleagues, who compose the very thin line of Agency officers working in the shadows at the very tip of the war on terror, are a national asset; an asset often misunderstood and under appreciated."

CIA Director Leon Panetta said in a statement Thursday that "those who fell ... were far from home and close to the enemy, doing the hard work that must be done to protect our country from terrorism."

A U.S. military source noted that Chapman was originally a base for the Khost Provincial Construction Team, but the team left some time ago. A U.S. intelligence official would not discuss the specific mission of the base but acknowledged that it was a crucial CIA post and a "hub of activity."

Authorities believe that the suicide bomber might have attacked just after a convoy was ending or beginning, which would account for the high number of casualties.

Eight Americans were originally believed to be killed in the bombing.



CNN's Barbara Starr, Pam Benson, Atia Abawi and Suzanne Simons and journalist Matiullah Mati contributed to this report.