The alarm about the accident came shortly after 10pm on Tuesday evening. According to the Swedish Maritime Administration, the wrecked recreational boat was in the company of another boat, and is suspected of having collided with a merchant ship.

"We have a picture of how it could have happened, but it's still a bit unclear. We need a little more information before we go public with it," said police spokesman August Brandt.

Four people were on the boat: a man and a woman in their 40s and two younger children.

A major rescue operation was launched, including resources from the Swedish Maritime Administration, emergency services, and the coast guard. Drones were used in the search and a helicopter was also involved.

The man and one of the children were found and taken to hospital with serious injuries, according to the police. The woman and a child are still missing. The darkness and the weather meant that the search had to be paused during the night, just after 2am.

"What we know is that we have found wreckage and that the boat has sunk," said Niclas Härenstam, communications director at the Swedish Maritime Administration.

A preliminary investigation into negligence in maritime traffic has been initiated.

"That is the classification that is closest to hand based on the information we initially have. But it may change depending on what we find out. No one has been notified of being suspected of a crime at this time," said August Brandt of the police.

The search resumed on Wednesday morning.

"In order to get a better overview of a larger area again, the plan is now to go up with the helicopter," said Niclas Härenstam of the Maritime Administration.

The merchant ship suspected of having been involved in the accident has been stopped.

According to the police, the ship involved is now just south of Tjörn, a large island off the coast of western Sweden. The police are on site on the ship for a technical examination. Interviews have been held overnight and will continue throughout the day today.