Interior Minister Ivan Demerdzhiev said the authorities "are not ruling out outside interference" in the fire, which comes after a similar incident at another EMCO warehouse last month.
EMCO, whose weapons are supplied to Ukraine for use against Russian forces, said in a statement that the storage facility had not been used for months and was inspected just 20 days ago, when full compliance with all regulations was confirmed. The company said human error was "completely ruled out" and accused someone of sabotage, though it did not specify who might be responsible.
The firm cited a number of similar incidents at Bulgarian defence companies since 2011, describing the latest as "no exception to the series of acts of sabotage against the Bulgarian defence industry." It called on Bulgaria, as "the EU and Nato member state most severely affected by such actions," to join a collective effort to prevent and neutralise these attacks.
EMCO is run by Bulgarian arms dealer Emilian Gebrev, who Bulgarian prosecutors allege was targeted in a 2015 poisoning attempt against him, his son and another employee. Three Russian nationals were later charged in absentia for attempted murder.
Bulgaria has been a major weapons supplier to Ukraine since the start of Russia's full-scale invasion in 2022. Although the government has recently halted state-owned arms supplies to Ukraine, Bulgarian private defence companies continue to supply weaponry.