LONDON — The captain of the Solong cargo ship who was arrested after crashing into a US-flagged tanker off the English coast is a Russian national, the vessel’s owner said Wednesday, as maritime experts search for answers.

The Solong careered into the Stena Immaculate while it was at anchor in the North Sea and carrying huge amounts of jet fuel for the US military, setting fire to both vessels and prompting emergency rescue efforts by the British coastguard.

British police have since opened a criminal investigation into the crash and arrested a 59-year-old man on suspicion of gross negligence manslaughter.

A spokesperson for Ernst Russ, which owns the Solong, confirmed to CNN that the arrested man is the ship’s captain and that he is a Russian national.

The rest of the crew are a mix of Russians and Filipinos, the spokesperson added.

Martyn Boyers, chief executive of the nearby Port of Grimsby East, had expressed disbelief that such a crash could have happened, given the sophistication of modern shipping technology.

“It seems a mystery, really, because all the vessels now have very highly sophisticated technical equipment to plot courses and to look at any obstacles or anything they’ve got to avoid,” Boyers told CNN on Monday.

“How did that vessel (the Solong) continue plowing into the berth vessel? There must have been some warning signs. They must have been able to detect it on the radar,” he said.

The Portuguese-flagged Solong was still burning more than a day on from the crash, while the fire on the Stena Immaculate was put out earlier Tuesday.

A missing crew member from the Solong is presumed dead, according to Britain’s maritime minister Mike Kane, after a search and rescue operation was called off late Monday.

The cargo ship’s other 13 crew members were rescued, along with the full 23-person crew of the Stena Immaculate which, Kane said, was carrying 220,000 barrels of jet fuel.

The Stena Immaculate, which is managed by the United States logistics firm Crowley, is part of a fleet of 10 tankers involved in a US government program to supply its military with fuel. The Department of Defense’s “Tanker Security Program,” according to Crowley, “ensures a commercial fleet can readily transport liquid fuel supplies in times of need.”

Oceanu UK, an environmental group, warned of “huge” potential damage if jet fuel spills into the sea. The fuel “can cause a huge amount of damage to wildlife as well as fish populations,” they said.

Environmentalists were also concerned by reports Monday evening that the Solong was carrying large amounts of sodium cyanide. But Ernst Russ later denied the claims, saying that the ship was carrying four empty containers that had previously contained the hazardous chemical. — CNN