Between 11 and 15 survivors from the crew of the Chelsea 3, the ship attacked in the Black Sea off Byala, have been questioned before a judge. Supervising prosecutor Kiril Peychinov told BTA that an explosives expert report has also been ordered.

Defence minister Dimitar Stoyanov told reporters today that Burgas port is closed because of the probe into the drone attacks. He said the debris found has yet to be identified.

See also: Burgas port closed over investigation into strike on Chelsea 3

Officers from the Special Counter-Terrorism Unit (SOBT) took swabs from the holes in the hull. The report must establish the type and amount of explosive, and whether it has any distinctive features.

Fragments found yesterday have gone for a separate expert check, which will look for serial numbers or marks showing where the drone came from. It is not clear whether they come from the ship or from a naval drone. Peychinov said they should not be called "drone parts".

"At first inspection these are not parts of the ship, but this expert check has been ordered to be sure," the prosecutor said. The results will not be ready for a week or two at the earliest.

All the operational services are taking part in the probe. Peychinov said foreign partners help as specialists but do not investigate. There is one investigation, he said, and Bulgarian authorities lead it. He added that a second opinion is welcome, because Bulgaria has not faced such a situation before.

The Sofia Military District Prosecutor's Office opened pre-trial proceedings yesterday after a report of a crime linked to the Chelsea 3. The incident happened in Bulgaria's exclusive economic zone in the Black Sea. The Sofia District Prosecutor's Office supervises the case.

The Malta-flagged motor ship was attacked 17 nautical miles east of Byala, and all 15 crew members were rescued. The other vessel hit, the HS Dignity, sails under the flag of Guinea-Bissau. A drone struck it and it sank 17 nautical miles east of Shabla.

Prime minister Rumen Radev said on 13 October that there had been new drone attacks on ships off Bulgaria.