By Tim Lister and Angela Dewan CNN

MOSCOW (CNN) -- Nine people have been killed in a blast on the St. Petersburg metro, Russian state media reported, in what authorities described as a terrorist attack.

An explosion tore through a train as it was traveling between two stations in Russia's second-biggest city. Around 50 people were injured, the governor's office in St. Petersburg said.

A spokesman for the National Anti-terrorism Committee said that the blast was caused by an unidentified explosive device in one of the cars.

"So far, we say it was an unidentified explosive device as investigators and the Federal Security Service's bomb specialists are to establish the exact cause of this explosion," Andrei Przhezdomsky told state-run Russia 24.

A device was found in a second St. Petersburg metro station -- Revolutionary Square -- but was made safe, the committee said in statement.

The Prosecutor General's office in St. Petersburg called the explosion a terrorist attack, TASS reported.

The explosion took place as the train was in a tunnel between the Sennaya Ploshchad and Tekhnologichesky Institute stations. In the confusion, first reports suggested there were two blasts.

Images of the car shows the facade had been ripped off, while others showed passengers running from the site at the station filled with smoke.

'We expected death'

Victims said they helped each other escape the train and that they were preparing for the worst possible situation.

"In the metro car, everyone expected death, if I can say that. After the explosion, everyone expected consequences. Then we were taken out, and people began to help each other, brought others out. Most were covered in blood," a passenger on the train told TASS.

Another passenger at the Sennaya station, Stanislav Listyev, said he felt the explosion and saw smoke coming out of the tunnel.

"I was going going down the escalator at Sennaya square at about half past 2, and at that moment I felt an explosion wave underneath. Everything was filled with smoke, people started panicking. So the trains stopped and almost immediately the evacuation started," he told CNN.

The source who confirmed the explosion to TASS said preliminary information indicated that around 10 people were killed, adding that the exact number of victims was still being established. The death toll was later revised to nine.

"At the moment there are about 50 injured, doctors are working with them. The number of ambulances there at the moment is 17. It will increase," said the governor's press secretary, Andrei Kibitov.

Russian President Vladimir Putin expressed his condolences to the victims and is talking to the FSB security services about the investigation, according to state media.

He said he was not ruling out any causes.

"The reasons for the explosion are unknown, so it's too early to talk about it. The investigation will show what happened," Putin said, beginning a meeting with Belarussian President Alexander Lukashenko.

"Naturally, we always consider all options -- both domestic and criminal, primarily incidents of a terrorist nature."

Kremlin spokesman Dmitry Peskov said Putin had been in St. Petersburg speaking at an event earlier Monday.

The St. Petersburg metro has five lines and 67 stations. It transports 2.3 million people a day and has more than 1,600 train cars.

What happened?

The Kremlin would not speculate on the motive behind the attack.

But Viktor Ozerov, chairman of the defense committee in the Federation Council, said that the choice of St. Petersburg as a target may have been tied to Putin's visit there for a media event.

"The choice of the place and the timing of these blasts is not accidental. The president of Russia is in [St. Petersburg], the media forum is taking place there, there are many journalists," Ozerov said.

Russia was once a hotspot for terror attacks but the country has experienced relatively few in recent years.

In 2010, two female suicide bombers blew themselves up at two Moscow metro stations, killing 40 people. They were linked to the Chechen insurgency.

In 2002, Chechen rebels killed 170 people in a theater hostage situation in the capital, Moscow.

CORRECTION: An earlier version of this story attributed the terrorism motive to the Russian prosecutor general. It was the city's prosecutor general.

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