COPENHAGEN, DENMARK — A Danish amateur submarine and rocket builder known as "Rocket Madsen" has been arrested over the disappearance of a Swedish journalist who was reportedly on his submarine before it sank.

The missing journalist has been identified as freelance journalist Kim Wall, who was last seen on board the UC3 Nautilus submarine with the owner Peter Madsen. The submarine left the port of Copenhagen on Thursday evening.

According to local media DR, Wall's boyfriend reported her missing on early Friday morning when they didn't return, which prompted a search-and-rescue operation.

On Friday morning, private motorboats discovered the submarine near the Drogden Lighthouse in Køge [c]Bay. Rescuers said he jumped into the water and swam for safety as the submarine sank, DR reported.

After he was brought to shore, Madsen told police that the submarine sank due to problems with a ballast tank. He also claimed that he dropped off Wall at the port of Copenhagen on Thursday night.

Copenhagen police conducted an examination of the submarine from Saturday to Sunday and determined that it was sunk deliberately, contrary to Madsen's claim of a technical fault. However, no body was found in the boat.

The crowdfunded UC3 Nautilus is reportedly the largest privately-built submarine in the world. Wall's family said the journalist had been with Madsen for an interview for a story on the vessel.

Madsen has been charged with involuntary manslaughter but has denied any wrongdoing.