A blast at a military plant in central Bulgaria killed one and injured four others on Saturday, the interior ministry said in a statement.
The ministry said the accident occurred during a planned experimental production test at the military complex in the village of Anevo – part of the state-owned arms producer VMZ Sopot.
Bulgarian national radio and local media reported that the man killed and two of the injured were US citizens. There were also unconfirmed reports that two Bulgarians and a Canadian were wounded.
The injured are being treated at the Sv Georgi University Hospital in the city of Plovdiv.
Plovdiv prosecutors and members of the labour inspectorate were at the scene of the explosion and a mobile team of the National Investigative Service is on the site.
Interior Minister and Deputy Prime Minister Roumyana Buchvarova also visited the scene, according to the Sofia Globe.