The damaged door of a train car is seen after an explosion on the subway in St. Petersburg, Russia, on Monday, April 3. Multiple people were killed in the blast. Russian Look via ZUMA Wire

Smoke fills a subway station near the blast. The explosion, which authorities described as a terrorist attack, took place on a subway car as the train was in a tunnel between the Sennaya Ploshchad and Tekhnologichesky Institut stations. Fontanka Ru/YouTube

The damaged train car is seen at the Tekhnologichesky Institut station in St. Petersburg. Dozens were reported injured in the blast. DTP&ChP St. Peterburg via AP

The aftermath of the explosion is evident at the Tekhnologichesky Institut subway station. www.vk.com/spb_today via AP

A person is rushed away from the scene on a stretcher. Russian Look via ZUMA Wire

A woman makes a phone call at the entrance to the Tekhnologichesky Institut station. Kovalev Peter/TASS via ZUMA Press

Emergency workers are seen outside the entrance to the Tekhnologichesky Institut station. Peter Kovalev\TASS via Getty Images

A man with bleeding hands stands outside the entrance to the Tekhnologichesky Institut station. Vaganov Anton/TASS via ZUMA Press

An emergency-response helicopter takes off near the Tekhnologichesky Institut station. Kovalev Peter/TASS via ZUMA Press

Firefighters and ambulances respond at the Sennaya Ploshchad station. Sergei Konkov\TASS via Getty Images