SOFIA: Three U.S. government defense contractors caught in a blast at a Bulgarian military range were testing out arms for purchase as part of the U.S.-led mission to equip Syrian opposition fighters battling ISIS, a U.S. military source said 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 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.

According to a U.S. military official who was speaking on condition of anonymity, the U.S. contractors were preparing a deal to buy arms for the U.S.-led effort to train moderate opposition fighters to battle ISIS in Syria.

“These guys were out there prepping [arms] for eventual transfer … for the Syria T&E (train and equip) effort,” the official told Reuters.

The official said the arms were being screened by the contractors for their usability. He declined to provide further information.

The Bulgarian government press office was not immediately available for comment Tuesday, while the defence ministry spokesman said he was unaware of this information and could not comment.

The two U.S. Department of Defense contractors who were injured in the blast were taken to a U.S. military hospital in Germany, according to a spokeswoman for the hospital where the injured were initially taken in the city of Plovdiv.

Bulgaria’s economy ministry has said the range was hired by Bulgarian private arms company Alguns, which also owned the grenade, produced in 1984 by a Bulgarian military plant. But the ministry also said it had not issued Alguns with a permit to export or re-export arms such as the grenade that malfunctioned.

No one was immediately available for comment from Alguns. The U.S. embassy in Sofia declined to comment.