COPENHAGEN — Did a Danish submarine owner kill his only passenger, a Swedish journalist who had accompanied him onboard for a story?

The question is gripping the public’s imagination and making headlines in Denmark and Sweden.

The inventor, Peter Madsen, an amateur space rocket and submarine builder known here as “Rocket Madsen,” was arrested and jailed on charges of involuntary manslaughter, according to the local news media, even though the journalist, Kim Wall, 30, remains missing.

On Sunday, after investigators entered the vessel — which had sunk but was raised — the police said there was no body inside. They also said the defendant had given different explanations about what had happened onboard.

“It appears to be a deliberate act to sink the submarine,” the chief homicide investigator, Jens Moller of the Copenhagen police, said at a news conference.