Kim Wall has reported from the heart of postwar Sri Lanka and from the capital of North Korea. While covering climate change in the Marshall Islands, she was quarantined and tested for exposure to radiation.

The 30-year-old freelance journalist, based in New York and China, is known among her friends as an intrepid reporter, skilled at exploring hard-hitting topics in obscure, and at times dangerous, locations.

So to her loved ones, a reporting trip off the coast of Denmark appeared to be a relatively safe destination. It was, after all, less than 30 miles from her Swedish hometown and in a country that ranks among the world’s safest.

But it was on this trip, aboard a submarine, that Wall vanished late last week.

She was last seen Thursday, departing from Copenhagen on the vessel along with its owner, a Danish inventor and amateur rocket builder named Peter Madsen, 46. According to her family, Wall was working on a story about Madsen, who is well known in Denmark for using crowdfunding to build his own submarines and rockets.

Copenhagen police have arrested Madsen and detained him on a charge of involuntary manslaughter, although no body has been found, according to a police news release. As the search for Wall continues, police also said they would like to talk with anyone who may have seen her leave the submarine.

Police say Madsen deliberately sank the submarine on Friday, when he was plucked from Koge Bay. Officials lifted the vessel out of the bay, where it had sunk more than 22 feet.

“The sub has been searched and there is nobody on board — neither dead nor alive,” Copenhagen police homicide chief Jens Moller told reporters at a news conference, according to Reuters and other news outlets.

Madsen has denied the charge, saying he dropped Wall off in the harbor of Copenhagen late Thursday after she completed her reporting. Madsen has since changed his story, police said Sunday, but declined to explain further.

Madsen’s defense lawyer, Bettina Hald Engmark, told the Associated Press her client is “willing to cooperate” and hasn’t decided whether to appeal the ruling on his detention. After a two-hour private hearing Saturday, a judge ordered that Madsen be held for 24 days while police continue investigating.

Police had been searching for the submarine, named the UC3 Nautilus, since Friday morning, when Wall’s boyfriend reported that she had not returned to Copenhagen Thursday night as planned.

Krisitan Isbak told a Danish news outlet that he spotted the submarine, and Madsen in the vessel’s tower, after authorities asked for help in the search. He saw Madsen go down into the submarine and reemerge shortly after. Then the vessel began to sink, he said.

“There was no panic at all,” Isbak said. “The man was absolutely calm.”

Isbak described the scene further to the AP, saying, “there was then some kind of air flow coming up and the submarine started to sink.” Madsen stayed in the tower until water began pouring into it. As the boat sank, he swam to a nearby boat, Isbak told the AP.

Madsen spoke to Danish television station TV2, claiming the submarine sank after “a minor problem with a ballast tank,” which holds water to provide stability, “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 the station. “But I guess that was pretty good because I otherwise still would have been down there.”

The UC3 Nautilus is described on its website as “one of the world’s largest home-built submarines” and launched in May 2008 in Copenhagen’s harbor. In 2014, Madsen began a new endeavor, a space laboratory that relies on donations. It aims to be the first non-governmental, fully volunteer-driven organization to launch a human into space, according to the BBC.

This photo allegedly shows Swedish journalist Kim Wall standing in the tower of the private submarine “UC3 Nautilus” in Copenhagen Harbor. (Anders Valdsted/AFP/Getty Images)

Wall’s family declined to speak in detail about her disappearance. But her mother told The Washington Post, “we still hope for her safe return.”

“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,” the family wrote in a statement to the Committee to Protect Journalists. “We sincerely hope that she will be found and that she is well.”

Wall’s disappearance resonated among journalists around the world, with many of them sharing the news across social media over the weekend. Her reporting covered topics such as identity, gender, pop-culture, social justice and foreign policy, and her work has appeared in Harpers, the Guardian, the New York Times, Foreign Policy, the Atlantic, and many other publications.

A native of Malmö, Sweden, Wall graduated from Columbia University with master’s degrees in journalism and international affairs. She received a bachelor’s degree from the London School of Economics Political Science.

The International Women’s Media Foundation described her as a “beloved member” of its community and said it was “enormously concerned” about her disappearance.

“We ask that the Danish authorities urgently make every effort to locate Kim and provide everyone who loves her with more information,” the organization wrote in a statement. “The global press freedom community is united in standing with Kim, her family and colleagues.”

The Committee to Protect Journalists echoed these demands, calling for an exhaustive inquiry into Wall’s “fate and wherabouts.”

“Denmark should not be considered a dangerous assignment for journalists,” wrote Nina Ognianova, the group’s Europe and Central Asia program coordinator.

Christopher Harress‏, a reporter for AL.com based in Alabama, told The Washington Post he became good friends with Wall while attending Columbia University with her. She was in all of his classes, he said.

After she graduated in 2013, Wall’s peers admired her decision to pursue a career as an independent journalist, move to China and chase stories around the world. She often relied on grants to fund her reporting trips.

“She did it all,” Harress said. “She’d been to all these different dangerous places.”

So when Harress and Wall’s other classmates heard Wall was missing, they initially presumed she may have simply wandered to pursue a story on an island.

“Because it is such a safe country, we just thought off course she’s gone somewhere,” Harress said. “We just thought that was her, that was what she did. She just wandered places.”

But as the news reports took a more ominous turn, her friends became increasingly worried.

“I think we just fear the worst now,” Harress said. “She trusted somebody and then this is what happened.”

He said her disappearance has “shaken up” many of Wall’s friends and fellow journalists, and has “tested a lot of our perceptions of what can happen to someone in certain places.”

“You can go to Africa and be perfectly safe and then go to one of the safest places in Europe,” and this happens, he said.

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