(CNN) -- An attack by a suicide bomber at a military base in eastern Afghanistan on Wednesday killed eight Americans believed to be CIA employees, a senior U.S. official told CNN.

Also Wednesday, four Canadian soldiers and a Canadian journalist were killed when a roadside bomb hit their armored vehicle in southern Afghanistan, Canada's defense ministry said.

The suicide bombing happened at Forward Operating Base Chapman in Khost province, Afghanistan. The senior U.S. official who spoke to CNN said information indicates the bomber walked into a gym facility at the base and detonated a suicide vest. It is believed six Americans were wounded in addition to the eight killed. It's not known how the bomber got past security.

A U.S. military source said that FOB Chapman was originally a base for the Khost Provincial Construction Team, but the team left some time ago.

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

"FOB Chapman is definitely more than a listening post. You can land helos [helicopters] there," the source said.

Meanwhile, the attack that killed the five Canadians happened about 4 kilometers (2.5 miles) south of Kandahar, where the Canadian contingent in Afghanistan is headquartered.

"The soldiers were conducting a community security patrol in order to gather information on the pattern of life and maintain security in the area," Brig. Gen. Daniel Menard, the commander of the 2,800-member Canadian contingent, told reporters. "The journalist was traveling with them to tell the story of what Canada's soldiers are doing in Afghanistan."

Four other Canadian troops and a civilian official also were injured in the attack, he said.

The Calgary Herald identified the reporter as Michelle Lang, 34, who had been with the paper since 2002. Lang is the first Canadian journalist killed in the Afghan war and is believed to be the first Herald reporter killed while on the job.

The deaths bring the number of Canadian military fatalities in Afghanistan to 138. The names of the troops were not immediately released.

The deaths are the most Canadians killed in a single incident in Afghanistan since six Canadian soldiers died in a bombing on July 4, 2007.



CNN's Barbara Starr and Atia Abawi contributed to this report.