WASHINGTON (AP) — A U.S. investigation has concluded that the killing of eight U.S. airmen and one American civilian contractor in Afghanistan last spring was a premeditated act by an Afghan colonel acting alone.

The investigation by the Air Force Office of Special Investigations was released Tuesday.

It probed the circumstances around the April 27 attack at Afghan air force headquarters at Kabul International Airport. Seven U.S. Air Force officers, one Air Force female enlisted member and a U.S. civilian were shot to death.

Investigators found evidence that Col. Ahmed Gul had followed the teachings of the Taliban and was upset at the presence of foreigners in Afghanistan.

The probe also found that in at least two earlier instances Gul had said he "wanted to kill Americans."