Military prosecutors in Sofia opened a pre-trial investigation on 29 October into a sea drone strike on the Liberian-flagged container ship CONTSHIP JET, the prosecutor's office said. They suspect banned methods of warfare were used.

Three sea drones hit the ship around midday on Wednesday, and it sank soon after. The strike was off Kaliakra, in Bulgaria's exclusive economic zone. Some of the crew survived.

How it all started: Sunken container ship off Kaliakra poses no threat to coast for now, Maritime Administration says

Prosecutors say an order was given for a drone attack on a ship of a neutral state, including its crew and cargo. That points to a suspected breach of the international rules of war.

The case began with a report from the Military Police regional office in Varna. Because the case is complex, a military investigator from the prosecutor's office in Sofia is leading the inquiry.

The surviving sailors are foreigners and are for now in Bulgaria, so Military Police in Varna must question them at once.

Investigators still have to establish who owns the ship, what cargo it carried, who was in the crew, what route it was sailing and how much damage the strike caused.