COPENHAGEN — The mysterious disappearance of a Swedish journalist who vanished after boarding a Danish inventor’s submarine took a dark turn Monday as police revealed the inventor had changed his account, telling them she died on his vessel and he buried her at sea.

The inventor, Peter Madsen, is being held on charges of involuntary manslaughter.

Initially, he told authorities he and the journalist, Kim Wall, had gone out on Aug. 10 in the UC3 Nautilus, a 26-foot submarine he built, and he dropped her off in a remote part of the port of Copenhagen that night.

The next morning, her boyfriend reported her missing, and Madsen was arrested. A search found the sunken vessel in Koge Bay, south of Copenhagen, after Madsen had plunged into the water and swam toward a boat, his rescuer, a private citizen, said.

Madsen now says “an accident happened onboard the submarine which lead to Kim Wall’s death and that he subsequently buried her at sea,’’ Copenhagen police said Monday.

The new account appears to raise more questions than it answers: If there was an accident, why didn’t Madsen call police? Why didn’t he bring her body to shore? Why did he initially say he had dropped her off on land?

The submarine was brought ashore shortly after it sank.

Using divers and sonar, the authorities were searching for Wall’s body on the sub’s route.

Madsen’s new account was disclosed Monday, with approval of the prosecution and the defense, though he had given it in court, behind closed doors, Aug. 12, a day after his arrest.

Madsen’s lawyer, Betina Hald Engmark, told the Danish television network TV2 her client was cooperating with police and maintains he’s not guilty.

The details were not immediately made public, officials said, to protect the police investigation and out of concern for Wall’s family. Her relatives have said they believed Wall, 30, had traveled to Denmark on a reporting assignment.

Friends of Wall have told the news media she was about to move to China with her boyfriend. A freelance journalist and a graduate of Columbia University, she had written for a wide range of publications, including The New York Times.

Madsen, 46, is known in Denmark as “Rocket Madsen,’’ a builder of submarines and space rockets who was hoping to become the world’s first amateur space traveler riding in a homemade rocket. For years he was able to build a community that offered helping hands and raised funds for his projects. But his temper caused conflicts with many of them, Thomas Djursing, a biographer, told BT, a Danish newspaper.

“He argues with every Tom, Dick and Harry,’’ Djursing said. “I’ve argued with him as well. But that’s what it’s like with people driven by deep passion.’’