A gunman opened fire in a Moscow government services center on Tuesday, killing two people and injuring four, Russian authorities said. The shooting reportedly began after the suspect was told to put on a face mask, according to a state news agency. A 45-year-old man, whom several Russian agencies reported was a former military serviceman, was detained shortly after the incident. His identity and possible motivations have not been disclosed.

Moscow Mayor Sergei Sobyanin confirmed the suspect’s arrest on Twitter, adding, “Doctors are doing all they can to help those wounded.” Deputy Mayor Anastasia Rakova identified those killed as an administrator and a security guard. In a post on messaging platform Telegram, Rakova said that three wounded adults were being operated on, and one 10-year-old girl was in “serious” condition. Russian news agency Tass reported the man pulled out a firearm after a heated argument with the guard over a mask. Officials are looking into charges of murder and illegal arms trafficking.