Copenhagen police said Tuesday they were scanning an inventor's homemade submarine for possible hidden rooms as they investigate the death of a Swedish journalist last seen on board before her headless torso was found.

Peter Madsen, 46, who has been held in formal custody since August 12 and accused of "negligent manslaughter", has insisted that Kim Wall died in "an accident" aboard his submarine and that he later "buried her body at sea". 

Police said in a statement they were inspecting the 60-foot (18-metre) Nautilus submarine with a cargo scanner to "search for clues to the crime", as well as possible weapons and hidden rooms. 

Police said they wanted to "to rule out the possibility that there may be unsearched rooms" aboard the craft.

The vessel was initially searched on August 13 and traces of Wall's blood were found inside, but no body. 

Wall's cause of death is not known, nor is the possible motive.

A search for the rest of her remains continues.

Wall was a 30-year-old freelance journalist who met up with Madsen to interview him for a feature story.

Wall, Madsen and the submarine were reported missing early on August 11, after Wall failed to return from her interview the previous evening.

Madsen and the submarine were located several hours later. He was rescued, alone, from the sub just moments before the craft sank off Koge Bay, south of Copenhagen.  

Investigators believe Madsen, who denies killing Wall and mutilating her body, "deliberately" sank the vessel shortly before he was rescued. 

After Wall's torso was found in Danish waters on August 21, prosecutors said they would seek to keep him in custody on a murder charge. They have until September 5 to request an extension of his custody.

A graduate of Columbia Journalism School, Wall had reported for The New York Times and The Guardian, among others.