Otto Warmbier was arrested and jailed in 2016 after Pyongyang accused him of attempting to take a propaganda sign from a hotel while on holiday. He was released to his family in the US in June, but he was in a coma and died shortly afterwards without regaining consciousness. The cause of his death remains unclear. Image caption Otto Warmbier appeared in a news conference in 2016 confessing to stealing a propaganda sign The State Department said at least 16 citizens had been detained in North Korea in the past decade. Three are known to be in detention. Most of those detained have been US missionaries, journalists and professors. The US has in the past accused North Korea of detaining its citizens to use them as pawns in negotiations over its nuclear weapons programme. Tourism offers outside visitors a rare window into North Korean life, but tour groups are heavily regulated as well as monitored. It is also a lucrative source of income, with one estimate that it nets authorities up to $43.6m (£34m) a year.