Three U.S. government defense contractors caught in a blast at a Bulgarian military range were testing arms for use in a U.S.-led mission to equip Syrian opposition fighters battling Islamic State, a U.S. military source said on Tuesday.

One American contractor was killed and two more were injured when a rocket-propelled grenade malfunctioned at a military range near the central village of Anevo on Saturday. Two Bulgarians were also injured in the explosion.

Bulgarian prosecutors have opened an investigation into the incident but the authorities declined to say what the Americans were doing in Bulgaria, a NATO member. […]