Moscow (CNN) -- The death toll in a Moscow metro derailment Tuesday has reached 22, the state-run RIA Novosti news agency said.
Another 160 people are injured, of whom more than 120 were hospitalized, the Moscow health care department said. Of those, a few dozen were in critical condition.
The derailment took place during morning rush hour in the Russian capital. Three metro train cars derailed in a tunnel, the state-run ITAR-Tass news agency reported.
More than 200 people were evacuated, the Emergency Ministry said on its website.
Dozens of ambulances arrived at the scene, along with three helicopters, to help shuttle the injured to hospitals, ITAR-Tass said.
Moscow Mayor Sergei Sobyanin declared Wednesday as a day of mourning.
The cause is under investigation
It was not clear what caused the derailment, but the head of Moscow's Transportation Department said it was not a terrorist attack.
However, it's still subject to a criminal investigation as authorities seek to find out what's behind the crash.
In the moments afterward, many of those caught up in the derailment scrambled back up to the surface at the station west of central Moscow, some bleeding and dazed as they emerged.
The rescue operation had become a recovery operation by Tuesday afternoon as emergency workers sought to free the remaining bodies from within the mangled train cars.
Their efforts were made more difficult by the depth of Moscow's metro lines, on which roughly 11 million people travel daily.
The metro system -- the backbone of the city's public transit network -- has been the target of past terror attacks.
CNN's Alla Eshchenko reported from Moscow, and CNN's Laura Smith-Spark wrote from London. CNN's Phil Black contributed to this report.