At least four American soldiers were shot and killed in an apparent insider attack by an Afghan soldier at a base in northern Afghanistan on Saturday, a military official said.

“An Afghan soldier shot and killed four American troops inside the base,” said Abdul Qahar Araam, spokesman for the Afghan Army’s 209th Corps. “We are investigating.”

The U.S. military command in Kabul confirmed that it was “aware of an incident” at Camp Shaheen, which is the headquarters of the 209th Corps in the northern city of Mazar-i-Sharif, but did not provide more details.