UK police Friday again extended the detention of the captain of a cargo ship that struck a tanker in the North Sea, citing the “complexities” of the case.

The Russian captain was arrested Monday on suspicion of gross negligence manslaughter after his ship, the Solong, slammed into the tanker, the Stena Immaculate, setting both vessels ablaze.

The tanker, which had been chartered by the US military, was anchored off the coast of Hull in northeastern England at the time.

Police were granted two extensions on Wednesday and Thursday to allow more time to question the 59-year-old captain due to “the complexities of the incident”, the local Humberside police force said in a statement.

Police cited the location of both vessels at sea as one of the complications facing the probe. The fires on both ships for several days after the incident required a massive firefighting response.

While all crew onboard the US-flagged Stena Immaculate were rescued, one sailor from the Portuguese-flagged Solong is missing presumed dead.

The Russian embassy in London was quoted by TASS news agency as saying in a statement Friday that none of the five Russian crew members on the Solong had been hurt in the incident.

It also said Russian consular officials had spoken to the captain by phone, who had told them that “he feels good”.

Although the British government has ruled out foul play, investigators are still determining the causes of the crash. The Solong never deviated from its course and slammed into the Stena at 16 knots an hour.

Pockets of fire were still being reported on the deck of the Solong on Thursday evening, according to the UK Coastguard.

“Extensive lines of enquiry are continuing,” police said.

Salvage teams boarded the vessels on Thursday to carry out initial damage assessments.