MAZAR-E-SHARIF, Afghanistan — An Army Reserve staff sergeant is in stable condition after he was wounded while apparently fatally shooting an Afghan army trainer who opened fire Tuesday at a base in northern Afghanistan.

Two U.S. civilian contractors and an Afghan soldier were killed when the trainer turned from his shooting position at a firing range at Camp Shaheen and started to fire on friendly forces, military officials said. The trainer was then gunned down.Afghan officials said an argument had erupted during routine weapons proficiency training.

No information was released on the reservist or contractors, who were training Afghan soldiers, pending notification of their families.

Applicants for training jobs undergo a standard vetting process, according to National Training Mission-Afghanistan spokesman Lt. Col. David Hylton.

Recruits must provide their government identification card and must be vouched for by two people from their village, preferably an elder, mullah or government representative.Biometric information such as fingerprints is then used to check for any outstanding criminal complaints or whether the person is a known insurgent.

Investigators from the International Security Assistance Force and the Afghan Ministry of Defense are looking into the incident.

The U.S. soldier is in stable condition, and will be evacuated to Landstuhl Regional Medical Center in Germany.

He suffered a perforated small bowel and superficial wounds to his left hip and is being kept in a drug-induced coma to reduce the risk of pneumonia.

He was struck by either “a bullet fragment or part of his metal belt buckle,” which was shot through, said Army Dr. (Capt.) Travis Mason, who operated on the soldier at the clinic at Camp Marmal.

Camps Shaheen and Marmal are on the southern edge of Mazar-e-Sharif in Balkh province, in the relatively quiet north of Afghanistan.

Last week, an Afghan soldier shot dead three British soldiers in Helmand province..

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