REYKJAVIK, Iceland — It has been more than four months since thieves pulled off the biggest heist in the history of Iceland, and the police here still have no idea where the booty — roughly $2 million worth of Bitcoin-mining computers — is stashed.

A breakthrough in the case seemed imminent in early February when the authorities detained Sindri Stefansson, a 31-year-old man with a rap sheet that includes drug possession and burglary. Even though he hadn’t been charged, let alone convicted, the media tagged him the “mastermind” of the crime, largely because he was held in prison longer than any of the 11 suspects who were questioned.

Then, like the computers, Mr. Stefansson disappeared. For the next five days he was an international fugitive. After escaping from the prison — a feat that took surprisingly little effort, given the institution’s bare-minimum approach to security — he hopped a taxi to the country’s largest airport, where he boarded an early-morning flight to Stockholm. In a twist that seems borrowed from a cheesy caper film, the plane also carried Katrin Jakobsdottir, the prime minister of Iceland.

“We did not chat,” Mr. Stefansson said, calling from a prison near Amsterdam in his first interview since he was arrested two weeks ago in the Netherlands. Speaking by phone in a gloomy monotone, he said he had worn a baseball cap and avoided the gaze of everyone on the plane. “I kept my head down as much as I could.”