COPENHAGEN, Denmark >> The owner of an amateur-built submarine was arrested on suspicion of murder today 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 in a statement tonight that the man denied killing the missing woman and reported dropped her off on a redeveloped island in Copenhagen’s harbor about 3 ½ hours into their Thursday night trip.

The statement 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 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 today 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 making 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 vessel 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.”

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 told The Associated Press. “They were the only two on board yesterday.”

Copenhagen Deputy Police Inspector Jens Moller Jensen says 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.

The police statement said the submerged submarine was lying in seven feet of water, but divers had not been able to enter it safely as of tonight. They were hoping to tow it to port on Saturday and open it then, the statement said.

Madsen describes himself as an “inventrepaneur” on the website for his Copenhagen-based company. RML Spacelab is seeking to send people into space and is developing a rocket, floating launch pad and astronaut testing machine among other projects, the site says.

“My passion is finding ways to travel to worlds beyond the well-known,” Madsen wrote on the site. “I design and manufacture non-commercial extreme machines, employing teams of volunteering engineers and technicians to challenge the ordinary….”

“The undersea world is close, it’s beautiful, always just around the corner in Denmark. Diving, no matter the method, is very challenging and it*s technically difficult to go to beyond where rubber suits and scuba gear can take us.”

The Nautilus set out from Refshale Island, a former shipyard redeveloped and purposed as an entertainment and activity spot, at about 7 p.m. local time on Thursday, according to a police timeline.

Two helicopters and three ships combed the sea from Copenhagen to the Baltic Sea island of Bornholm after police learned just after 3:30 a.m. It was spotted by a lighthouse in Koge Bay, a seaport south of the city, at 10:30 a.m.

“A radio contact was established for the boat, which, according to the owner, was heading towards the harbor,” the police statement said. Half an hour later “the submarine suddenly sank and the owner was subsequently rescued on a private motorboat sailing him in port.”

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.

Lisa Leff contributed from London.