MOSCOW—A gas leak in a Siberian coal mine killed at least 52 people on Thursday, Russian news agencies said, including six rescuers who were sent down to try to bring out dozens of men in what was one of Russia’s worst mining disasters since Soviet times. The regional Investigative Committee said three people, including the director of the Listvyazhnaya mine and his deputy, had been arrested on suspicion of violating industrial safety rules. It said miners had suffocated when a ventilation shaft became filled with gas. State television said prosecutors believed there had been a methane explosion. The dead comprised 11 miners already confirmed killed, 35 who had been reported missing, and the six rescue workers. Dozens of people were treated in hospital, at least some of them for smoke inhalation. Four were in critical condition. The coal-producing region of Kemerovo, roughly 3,500 km (2,200 miles) east of Moscow, has …