Police are investigating the mysterious disappearance of freelance journalist and recent Columbia University graduate Kim Wall, who was last seen boarding a submarine in Copenhagen on Thursday along with Danish inventor and amateur rocket builder Peter Madsen. Now, Danish authorities are alleging that the vessel was “deliberately sunk,” and the inventor known as “Rocket Madsen” is facing charges of preliminary manslaughter, as the search for Wall continues.

The Washington Post reports that Wall, 30, who is based in New York and China, was last seen getting into the UC3 Nautilus submarine with Madsen, a self-described “inventrepreneur” and the subject of her latest piece. When the vessel—described as “the worlds [sic] biggest amateur built submarine,” on a crowdfunding website—did not return the following day, Danish authorities launched a hunt, eventually locating the submarine in a bay about 30 miles south of Copenhagen. Police searched the sub, and Jens Moller, head of the Copenhagen Police's homicide unit announced at a news conference Sunday that "There are no persons in the submarine, dead or alive," Reuters reports.

Wall’s boyfriend, Krisitan Isbak, was also involved in the hunt, and told a Danish news outlet that he saw Madsen in the submarine’s conning tower, then watched him go down into the submarine. He returned to the surface just as the submarine began to sink, the boyfriend said.

“There was no panic at all,” Isbak alleged, according to the Washington Post. “The man was absolutely calm.”

Isbak elaborated on the story to the Associated Press:

"He then climbed down inside the submarine and there was then some kind of air flow coming up and the submarine started to sink," Isbak said. "(He) came up again and stayed in the tower until water came into it" before swimming to a nearby boat as the submarine sank, he added.

Madsen, meanwhile, claims that he dropped off Wall on a nearby island a few hours after their expedition began. He says the submarine sank after “a minor problem with a ballast tank...turned into a major issue.”

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

A judge ordered that Madsen be detained for 24 days as police investigate the crime and search for Wall. The inventor’s attorney said that he’d be “willing to cooperate.”

Wall, a freelance journalist born in Sweden, graduated from Columbia University with a master’s degree in journalism in 2013, and previously studied at the Sorbonne university in Paris and the London School of Economics, according to the Washington Post. She’d written for the New York Times, The Guardian, The Atlantic, Harper’s, the South China Morning Post and Vice Magazine.

“It is with a great concern that we, her family, received the news that Kim is missing after an interview with Peter Madsen in Denmark,” Wall’s family wrote in a statement to the Committee to Protect Journalists. “We sincerely hope that she will be found and that she is well.”

Here's a video showing Madsen and his submarine: