Danish prosecutors said Thursday they would seek a murder charge against the maverick inventor who was the last person seen with Swedish journalist Kim Wall before her headless torso was discovered.

Peter Madsen, an engineer who won notoriety for building his own submarine, was the subject of a feature article by Wall. She was pictured boarding the vessel to interview the 46-year-old but then disappeared earlier this month.

In a chilling case, the reporter's decapitated body was found in waters off Copenhagen. Investigators say it was "deliberately" mutilated and weighed down with a metal object to try and avoid detection.

Madsen has been held in formal custody since August 12 on suspicion of "negligent manslaughter".

But having recovered Wall's body, a murder charge is now being sought.

"At latest on September 5, we will try to hold him in custody on a murder charge ... after her (dismembered) body was found," special prosecutor Jakob Buch-Jepsen told AFP.

They also want Madsen to undergo a psychiatric evaluation and the authorities have until September 5 to request an extension of custody.

The eccentric inventor has denied the allegations.

He initially said he had dropped Wall off alive on a Copenhagen island, but later changed his story to say she died in an accident on board and he had buried her body at sea at an unknown location in Koge Bay, south of the capital.

Wall, 30, was a freelance journalist who had reported for The New York Times and The Guardian. Her boyfriend reported her missing a day after the interview with Madsen.

That same day, Madsen was rescued from waters between Denmark and Sweden shortly before his submarine sank.

Investigators recovered and searched the sunken vessel, which police believe Madsen sank intentionally.

- 'My temper to blame' -

Described as fanatical and foul-tempered, Madsen's career has been punctuated by stories of professional fallouts and mood swings.

Madsen, reportedly married, has a reputation for histrionics and has angered many along his way,

Said to have few material possessions, he has lived on his submarine at times.

Supported by financial backers, he also earns money from speaking engagements.

Seen as part Carl Sagan and part Jacques Cousteau, Madsen championed private submarine construction and space exploration, most recently leading a grassroots effort to launch a rocket built by amateurs rather than governments or multinational corporations.

The first rockets he launched into space were the result of a partnership with a former NASA architect, Kristian von Bengtson.

But the two men parted company after an angry spat in 2014 and Madsen created his own space project, RLM Space Lab.

"I'm fully aware that my temper is to blame for Kristian's exit and I'm very sorry it has come to this," Madsen wrote in a statement at the time.

- 'Always on the road' -

Wall was a graduate of Columbia Graduate School of Journalism, based between New York and China.

She had planned to move to Beijing to pursue her career, according to her former class mate and close friend Yan Cong.

"Kim (was) always on the road, always working on new stories somewhere in the world, but she talked about how much she wanted to be based in Beijing," Cong, 26, said.

Danish police are searching for the clothes Wall wore on the submarine: an orange fleece, flowery skirt and white sneakers.

"We sent each other photos of us wearing the sneakers during reporting trips from different parts of the world," Cong said.

"I believe she was wearing them when she went missing."

Wall had covered news about earthquake-hit areas in Haiti, Idi Amin's torture chambers in Uganda and minefields in Sri Lanka.

"She gave voice to the weak, to the vulnerable and marginalised people,"Wall's mother Ingrid wrote in a Facebook post on Wednesday.

"That voice would have been needed much, much longer. But now that will not be so."

Source: AFP