COPENHAGEN, Denmark — The owner of an amateur-built submarine was arrested on suspicion of murder Friday after his vessel sank off Denmark’s coast and a journalist who had joined him for what was supposed to be a short voyage was reported missing, Copenhagen police said.

Police said Friday night that the man denied killing the missing woman and reported dropping her off on a redeveloped island in Copenhagen’s harbor about 31/2 hours into their Thursday night trip.

Police did not identify the submarine’s owner, Peter Madsen, 46, but the Danish inventor’s financing of the project through crowdfunding and first launch of the UC3 Nautilus in 2008 made headlines.

The 40-ton, nearly 60-foot-long vessel has been described as the largest privately built submarine of its kind.

Before his arrest, Madsen appeared on Danish television to discuss the submarine’s sinking and his rescue.

It was the journalist’s boyfriend who alerted authorities early Friday that the sub had not returned from a test run, police said.

“I am fine, but sad because Nautilus went down,” Madsen told Denmark’s TV2 channel, which aired footage of him getting off what appeared to be a private boat and giving a thumbs-up sign.

Madsen said “a minor problem with a ballast tank” – a compartment that holds water to provide stability – “turned into a major issue” that ultimately caused the submarine to sink.

“It took about 30 seconds for Nautilus to sink, and I couldn’t close any hatches or anything,” he said.

“But I guess that was pretty good, because I otherwise still would have been down there,” he said.

Swedish police said later in the day 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 in a statement.

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,” Danish navy spokesman Anders Damgaard said. “They were the only two on board yesterday.”

Copenhagen Deputy Police Inspector Jens Moller Jensen said investigators are looking for witnesses who may have seen the woman after the time Madsen reported she disembarked. She also was not identified by name.

Police said the submerged submarine was lying in seven feet of water, but divers had not been able to enter it safely as of Friday night.

They were hoping to tow it to port Saturday and open it then, police said.