Kabul, Afghanistan (CNN) -- Military authorities are investigating the shooting deaths of two U.S. civilians and an Afghan soldier by another Afghan soldier in the northern city of Mazar-e-Sharif Tuesday.
The shooter also died in the incident, and two other service members -- one Afghan and the other a member of international coalition forces -- were wounded, a statement from NATO's International Security Assistance Force said.
The incident occurred during routine weapons proficiency training at Camp Shaheen, the statement said. The shooter is believed to be an Afghan army trainer assigned to Camp Shaheen.
"Our joint investigation will find out how such an event could have occurred to help us prevent any future acts," said Maj. Gen. Zahir Azimi, spokesman for the Afghan Defense Ministry. "We wish to express our deepest sorrow for the deaths of our colleagues in this tragic event today."
Josef Blotz, spokesman for ISAF, said: "The actions of this individual are not representative of the thousands of men and women who train and fight side-by-side with ISAF everyday to protect their fellow citizens from insurgent brutality."