By Jan M. Olsen | Associated Press

COPENHAGEN, Denmark — The owner of an amateur-built submarine was arrested on suspicion of murder Friday after his vessel sank in Denmark’s waters 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 Friday night that the man denied killing the missing woman and said he dropped her off on an island about 3 ½ hours into their Thursday night trip.

The statement did not identify the submarine’s owner, Peter Madsen, 46, but his success financing his submarine project through crowdfunding and completing the UC3 Nautilus in 2008 made headlines.

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

Footage aired on Denmark’s TV2 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 TV2.

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 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,” navy spokesman Anders Damgaard told The Associated Press. “They were the only two on board yesterday.”

It was the woman’s boyfriend who alerted authorities the submarine was missing early 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 Damgaard later said the 40-ton, nearly 60-foot-long submarine had sunk.

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