The blast occurred on Tuesday at an Arsenal group factory in the central town of Kazanlak.
Three people died after an explosion at a Bulgarian arms and pyrotechnics factory on Tuesday.
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The blast occurred at an Arsenal group factory in the central town of Kazanlak, prosecutors said. Video footage on social media showed a large pinkish cloud above the plant.
A 55-year-old male worker on the production line was killed and the bodies of two female colleagues -- aged 43 and 53 -- were later found.
Another woman was seriously injured and remains in a critical condition in hospital.
Investigators initially believe the explosion was caused by a "failure to comply with safety regulations" around explosive materials.
"The workers from the neighbouring premises were evacuated. A large part of the workshop was destroyed," a statement read.
Authorities are also warning of fresh aftershocks at the site due to fresh explosions.
It is the fourth such explosion at a Bulgarian arms factory since 2018.