At least 10 people were killed in a bomb blast on a subway train in St. Petersburg, Russia on Monday.

Russian media outlets reported the train car explosion injured at least 50 others in what President Vladimir Putin called a possible act of terror.

Investigators are looking into all possible causes of the two separate explosions — one occurring at the Sennaya Ploshad station and the other at the Technological Institute station.

Officials closed all subway stations in the northern Russian city and evacuated passengers. A witness told local press:

"People were bleeding, their hair burned. My girlfriend was in the next car that exploded. She said that she began to shake. When she came out, she saw that people were mutilated."

Putin, who was visiting the city on Monday to speak to the Belarussian leader, gave his condolences to the families of the victims.

The Russian leader said security service heads are trying to establish the "unidentified object" that caused the explosion, explaining to Sky News:

"I have already spoken to the head of our special services, they are working to ascertain the cause (of the blasts). The causes are not clear, it’s too early. We will look at all possible causes, terrorism as well as common crime."

Photos from the Sennaya station showed a train car with doors blown off, while injured commuters appeared to be fleeing the smoke-filled station.

Our hearts go out to the families of the victims amid this tragedy. See photos of the aftermath (below), but some it may be disturbing:

There’s reportedly been an explosion in the St Petersburg subway in Russia pic.twitter.com/QfYhSEd6QH
— Ben Makuch (@BMakuch) April 3, 2017

Another video shows site of explosion at St Petersburg Metro, Moscow, Russia pic.twitter.com/j8qqQH2LiF
— Brandi Saari (@brandilmelb) April 3, 2017

There was an explosion in the Saint Petersburg metro. At least 10 people died. #rip #russia pic.twitter.com/s3e1xwlNJN
— Francesco (@Londonandreams) April 3, 2017