Several Moscow Metro subway cars derailed Tuesday morning, killing 20 and injuring over 160 people.

The derailment occurred on the Arbatsko-Pokrovskaya blue Metro line during morning rush hour. The driver of the train is alive and being treated in a local hospital. 

The press service of the Moscow Emercom branch reported that 1,100 people evacuated from the site of the accident and at least 50 of those injured were in grave condition. The dark blue line carries some 733,000 people daily

The cause of the accident remains unknown, but authorities say they believe it was not an act of terrorism. Some reports indicate that the cars jumped off the track after a power surge triggered an alarm.

"There can be several reasons: failed switch, rail subsidence, because there are construction works in a tunnel nearby. We will consider the possibility of the carriage itself being defective - anything that will emerge during the investigation,” the spokesman for the Russian Investigative Committee, Vladimir Markin said at a press conference. 

Authorities have declared Wednesday a day of mourning for those who passed away in the accident.