A potential murder investigation has been launched after a woman and her seven-year-old son plunged from a ferry into the sea.
His 36-year-old Polish mother jumped overboard in an attempt to save him, they added.
The two were winched to a helicopter and rushed to Karlskrona hospital in Sweden but were declared dead there.
When the alarm was raised, a large-scale rescue operation, involving a thermal imaging camera and both Swedish and Polish emergency services, was launched.
The Swedish ferry, the Stena Spirit, had been in the Baltic Sea, midway through its journey to Gdynia in Poland from Karlskrona in Sweden when the pair fell, officials said.
The country’s prosecution chiefs said on Friday they had launched a preliminary investigation in which the criminal charge was murder, but added that no suspect had been identified.
“The investigation aims to try to clarify what happened,” prosecutor Stina Brindmark said.
Polish police issued an appeal to passengers asking for information on how the accident happened.
The mother was in the water for 59 minutes before being picked up, and her son 66 minutes, Polish website Fakt reported.
It said resuscitation was started immediately but the two could not be revived.
Police questioned the crew of the ferry to reconstruct events, but officers have not made contact with anyone who saw what happened, according to Fakt.
Stefan Elfstrom, spokesman for Stena Line, said someone on the ferry raised the alarm.
“Items have been found that belong to these people, but not the people themselves,” he said of the woman and child. “Then the alarm went off, the crew looked at the surveillance cameras and saw that two people had fallen into the water.
“There is little information on whether there were relatives or people who knew the two people on the ferry. However, someone noticed that two people were missing.”
A German Nato helicopter reportedly plucked the mother and son from the water.
Confirming the deaths, spokesman Mariusz Ciarka said: “Unfortunately, in the morning we received information from the Swedish side that we have to pass on this terrible news to the family, because both the boy and the woman are dead.”
The Swedish Maritime Administration said the boy fell from a height of about 20 metres (65ft).
“First we received a report that one person had fallen overboard, then it turned out to be two,” said Lars Blom, chief of rescue operations at the Swedish Maritime Administration rescue centre.
Stena Line said it could not confirm that the mother jumped in to save her son, but said the company was cooperating with police and other authorities to try to explain what happened.