In this frame grab from video provided by the Russian Ministry for Emergency Situations, rescue teams work inside the tunnel in Moscow where subway train derailed Tuesday during rush hour, killing at least 20 people and sending 150 others to the hospital, many with serious injuries, Russian officials said. (Russian Emergency Situation Ministry/AP)Medical helicopters wait to transport injured people after the derailment. It was unclear what caused the train to derail. Lines of inquiry included a fault in one of the cars or the sinking of the roadbed, according to Vladimir Markin, spokesman for Russia's top investigative body. (Ivan Sekretarev/AP)The Russian capital's airports and transit systems have been a prime target for terrorists over the past two decades, but multiple officials vigorously dismissed terrorism as a possible cause. (Russian Emergency Situation Ministry/AP)Park Pobedy, where the derailment occurred, is Moscow's deepest metro station -- 275 feet below the surface -- which made the rescue particularly difficult. (Russian Emergency Situation Ministry/AP)Paramedics and firefighters carry an injured man out of the station. Over 1,100 people were evacuated from the train, which was stuck between two stations, in a rescue operation that lasted at least 12 hours. (Ivan Sekretarev/AP)Paramedics carry an injured woman from the station . President Vladimir Putin, who is traveling in Brazil, demanded a detailed investigation into the "reasons for the event" and asked the country's top investigators to open a criminal case, his spokesman told Russian news agencies. (Ivan Sekretarev/AP)
