A US soldier was reported killed Monday in the Western Afghanistan city of Herat, slain in a “green-on-blue” attack by an unidentified Afghan soldier during training.

The soldier’s motivations are unknown at this time, but he was said to have thrown a hand grenade at his trainers, and the US soldier was slain. Two other people, including an interpreter, were wounded in the incident.

The soldier is believed not to have any ties to the Taliban, but rather had some sort of “psychological problems.” Pentagon officials promised investigations into the incident.

Though the number of “green-on-blue” attacks has declined as NATO occupation forces have less contact with the Afghan military, the number of insider attacks, including green-on-green attacks among Afghan troops, continues to rise.