A man has been arrested on suspicion of gross negligence manslaughter in connection with the ship collision in the North Sea.
Humberside Police said the 59-year-old is in custody to allow enquiries to take place, and officers are talking to those involved to find out what happened.
The force added investigators have started a criminal probe into the cause of the collision between the Stena Immaculate and Solong off the coast of East Yorkshire on Monday, and are working with the Maritime and Coastguard Agency.
Ernst Russ, the owner of Solong, later confirmed the man detained was the ship's captain. The shipping firm called him "the master of the ship" - which is understood to be the same as a captain.
One person remains missing and is presumed dead after emergency services stopped their search on Monday evening.
The Marine Accident Investigation Branch is also involved in early work to determine what caused the two vessels to collide.
Read more: What we know about the collision
Senior Investigating Officer, Detective Chief Superintendent Craig Nicholson, said: "Humberside Police have taken primacy for the investigation of any potential criminal offences which arise from the collision between the two vessels.
"Extensive work has already been carried out, and we are working closely with our partners to understand what happened, and to provide support to all of those affected.
"Following enquiries undertaken by my team, we have arrested a 59-year-old man on suspicion of gross negligence manslaughter in connection with the collision, this follows the conclusion of search operations by HM Coastguard for the missing crew member of the SOLONG."
He added: "Our thoughts are with the family of the missing crew member, and I have appointed family liaison officers to make contact and provide support to the family.
"The man arrested remains in custody at this time whilst enquiries are under way, and we continue speaking with all those involved to establish the full circumstances of the incident."
Ernst Russ added in a later statement that it could confirm the master of the Solong "has been detained by Humberside Police in the UK".
"The master and our entire team are actively assisting with the investigations," the shipping firm said.
"Out of respect for the investigation and all involved we will not comment further at this time."
Read more:
Oil tanker collision could have lethal ecological impact
Sky News above North Sea ship collision
US tanker Stena Immaculate was anchored off the coast of East Yorkshire when the Solong crashed into it at around 9.45am on Monday.
Some 36 people were brought safely to the shore, but one person was reported missing from the cargo ship.
At around 11.20pm, the Solong started to drift southwards away from the crash site. HM Coastguard said in an update earlier on Tuesday that the ship is still alight.
Tugboats are in the vicinity to ensure it remains away from the coast
Transport Secretary Heidi Alexander said the container ship is expected to stay afloat, after a minister told parliament that it could sink earlier in the day.
She said she met with the Maritime and Coastguard Agency's chief executive Virginia McVea, and was "pleased to have been informed that early indications suggest that both vessels are now expected to stay afloat".
Ernst Russ also denied reports the vessel was carrying sodium cyanide and said: "There are four empty containers that have previously contained the hazardous chemical.
"These containers will continue to be monitored."
Stena Immaculate was carrying 220,000 barrels of Jet-A1 fuel in 16 tanks, at least one of which was ruptured, its operator also said.
(c) Sky News 2025: Man arrested on suspicion of manslaughter over North Sea ship collision is captain of cargo ship