While the UC3 Nautilus was going down, Isbak says "came up again, stayed in the tower until water came into it as the submarine started sinking". The inventor has denied wrongdoing and said that he dropped Wall-who was reported to be the only other person on board the submarine with him-off on an island close to Copenhagen Thursday night before the submarine sank, according to AP reports.
Copenhagen Police have appealed for witnesses who might have been at Refshaleøen at around 19:00 and 22:30 - the times Madsen claims his submarine left and returned. He told authorities he had been alone on the vessel. The missing woman has been identified as Kim Wall, a 30-year-old New York-based freelance journalist who has written for the New York Times, the Guardian and Vice Magazine.
However, when she failed to return home later that day, her anxious boyfriend contacted the authorities, which led to a full-scale search for the submarine in the early hours.
Peter Madsen was arrested Friday on preliminary manslaughter charges after his 40-ton, almost 18-meter-long (60-foot-long) submarine sank off Denmark's eastern coast.
The eccentric DIY enthusiast crowdfunded the renovation of his UC3 Nautilus sub earlier this year.
Madsen has denied responsibility for the fate of Ms Wall, saying she had disembarked before his vessel went down.
It was built like a post-World War II submarine, with a galley, crew bunks, officer's mass, bridge and engine room.
On board were Mr Madsen and a Swedish journalist, whose partner had alerted authorities to her absence on Thursday evening, Copenhagen police said in a statement.
Footage aired on Denmark's TV2 channel showed Madsen, 46, getting off what seemed to be a private boat and making a thumbs up sign as he walked away.
The woman was a journalist writing about Madsen and his submarine, Swedish and Danish media reported.
The Danish navy says it is searching for a 40-ton, almost 18-meter (60-foot) -long, privately built submarine in the waters off Copenhagen with at least two people on board. Madsen said "a minor problem with a ballast tank. turned into a major issue" that ultimately caused the vessel considered the largest privately built submarine of its kind to sink.
A judge is set to decide if the owner of an amateur-built submarine should be held in custody during an investigation into whether he is responsible for the disappearance of a Swedish woman who had been onboard his submarine that sank off Denmark's east coast.
Madsen "told us he had technical problems" when asked to explain why the submarine failed to respond to radio contact earlier in the day, Damgaard said. The Navy said it was spotted sailing but then sank.