An Afghan employee of the U.S. Embassy in Afghanistan opened fire Sunday evening in a CIA annex at the embassy, killing a U.S. citizen, a U.S. government official said.

Security personnel used a flash-bang round to stun the shooter and then killed him, the official said.

The gunman shot indiscriminately, and he is not believed to have targeted the victim, who was working for the U.S. government, the official said. People went running for cover.

One or two civilians suffered minor wounds, the official said, speaking to CNN on condition of anonymity due to the sensitive nature of the information and the ongoing investigation.

Investigators are looking into whether the shooter was a disgruntled employee or whether he was inspired by militants.

All Afghan employees go through a background security check and must pass through a metal detector each time they enter the embassy, the official said.

“We have no information as to whether the individual was authorized to carry a weapon or if he seized a weapon,” the official said.

Afghan troops guard the outer perimeter of the embassy, while embassy contractors and members of the U.S. military guard the inside.

Embassy spokesman Gavin Sundwall said the Afghan employee, a lone gunman, killed a U.S. citizen and wounded another. The wounded American was evacuated to a military hospital with non-life threatening injuries.

“The motivation of the attack is still under investigation,” he said.

The annex area is known as a place where embassy personnel both live and work, some in intelligence operations. A CIA spokeswoman declined to comment on the attack.

Afghanistan has been the site of several high-profile attacks of late, including strikes at the NATO headquarters and the U.S. Embassy in Kabul and the assassination of former Afghan President Burhanuddin Rabbani. Rabbani had been leading efforts for reconciliation talks.

The latest shooting is not believed to be related to any other recent attacks in Kabul, the U.S. government official said.

CNN’s Barbara Starr and Nick Paton Walsh contributed to this report.