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

The inventor, Peter Madsen, is being held on charges of involuntary manslaughter. He initially told the authorities that he and the journalist, Kim Wall, had gone out on Aug. 10 in his submarine and that he dropped her off back on land in a remote section of the port of Copenhagen later that night.

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

Mr. Madsen now asserts that “there was an accident on board which caused Kim Wall’s death and that he consequently buried her at sea,” the Copenhagen police said in a statement on Monday.