The Danish owner of a homemade submarine that sank south of Copenhagen has been arrested on suspicion of murdering a Swedish journalist who was a passenger on the vessel.

Police said the man had denied killing the missing woman and claimed he dropped her off on an island on Thursday night.

Officers did not identify the owner, Peter Madsen, but he appeared on Danish television on Friday to discuss the submarine’s sinking.

Deputy Insp Jens Moller Jensen said investigators were looking for witnesses who may have seen the woman after the time Madsen said she disembarked.

The submarine is lying in 7ft of water, but divers have not been able to enter it safely yet, police said.

Madsen (46) financed his submarine project through crowdfunding and completed the submarine, the UC3 Nautilus, in 2008.

Footage aired on Denmark’s TV 2 channel showed him getting off what appeared to be a private boat and making a thumbs-up sign as he walked away.

“I am fine, but sad because Nautilus went down,” he told TV 2.

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.

The ballast tank is a compartment that holds water, which is used as ballast to provide stability for a vessel.

“It took about 30 seconds for Nautilus to sink, and I couldn’t close any hatches or anything,” Madsen said. “But I guess that was pretty good because I otherwise still would have been down there.”

However, Swedish police said later on Friday they were investigating the whereabouts of a missing woman who had been on the submarine at some point.

“Whether the woman was on board the submarine at the time of her disappearance is unclear,” police said.

Assignment

The woman was a journalist writing about Madsen and his submarine, Swedish and Danish media reported.

“He told us that the journalist who also had been on board had been dropped off Thursday evening,” navy spokesman Anders Damgaard said. “They were the only two on board [on Thursday].”

The woman’s boyfriend alerted authorities that the submarine was missing early on Friday. Two helicopters and three ships combed the sea from Copenhagen to the Baltic Sea island of Bornholm.

The navy initially said the sub was “found sailing” south of Copenhagen, but Mr Damgaard later said the 40-ton, 60ft-long submarine had sunk.

Madsen “told us he had technical problems” to explain why the submarine had failed to respond to radio contact, Mr Damgaard said.

AP