A woman's headless, limbless torso was found in Copenhagen as Danish police investigate the disappearance of journalist Kim Wall, who they believe to be dead. The 30-year-old was last seen on board a submarine built by the Danish inventor Peter Madsen, 46, who has been charged with her manslaughter.

Investigators believe Madsen deliberately sunk his NC3 Nautilus submarine, paid for by a crowdfunding campaign, off the Danish coast during his trip with Wall, who was writing a story about him and the vessel.

Madsen, who had to be rescued from the water, originally told police Wall had got off the submarine at an island around three hours into their journey. However, he has now told them Wall accidentally died in an accident on the submarine and he buried her at sea "at an unspecified location somewhere in Køge Bay", said police. Køge Bay is about 30 miles south of the Danish capital.

A cyclist in Copenhagen discovered a woman's torso by the river on 21 August. It is unclear if the body is Wall's, but it is being pursued by investigators into her death as a line of inquiry, and has been sent for forensic analysis. "It is clear that the police, like the media and everybody else, is speculating whether this female body is Kim Wall, but it is way too soon to tell," a spokesman for the police told a press conference.

In a previous statement, Prosecutor Louise Pedersen said Madsen faced a preliminary manslaughter charge "for having killed in an unknown way and in an unknown place Kim Isabell Frerika Wall of Sweden sometime after Thursday 5pm."

Betina Hald Engmark, Peter Madsen's lawyer, said her client has confessed to nothing and denies the charges against him.

Search operations by Danish and Swedish authorities to recover Wall's body continue. They started after Wall's boyfriend raised the alarm on Friday 11 August, the day after she went on the trip with Madsen.

Madsen's Nautilus, built in 2008, was the biggest private submarine ever made at the time.