A member of the European monitoring mission in eastern Ukraine was killed and two others were injured Sunday when their vehicle drove over a mine near Luhansk. The Organization for Security and Cooperation in Europe, which runs the monitoring mission, said it was investigating.

The monitor who was killed was an American man, according to a spokesman for the Foreign Ministry of Austria, which holds the rotating presidency of the 57-nation organization. The man’s name was not released.

The mission has had more than 700 unarmed civilian monitors deployed in eastern Ukraine since 2014. Their role now is to keep track of whether government forces and Russian-backed separatists in the region are complying with a 2015 cease-fire agreement, including commitments to withdraw heavy weapons.

The organization confirmed that the episode involved a patrol of six monitors in two armored vehicles.