Ten people were killed and dozens more injured Monday when an explosion rocked a subway train in the Russian city of St. Petersburg, authorities said.

Russian President Vladimir Putin, who was in Russia's second-largest city at the time, offered condolences to families and loved ones of those killed.

“The causes of this event have not been determined yet," Putin said. "Certainly, we will consider all possibilities: common, criminal, but first of all of a terrorist nature.”

Several metro stations across the city were shut down following the blast. Photos and video posted on social media from the scene showed a smoke-filled platform, a damaged subway train and people lying on the ground.

"So far, we say it was an unidentified explosive device," Andrei Przhezdomsky, a spokesman for the National Anti-Terrorism Committee, told Rossiya-24 TV. He said the Federal Security Service’s bomb specialists were working to establish the exact cause of this explosion."

Andrei Kibitov, spokesman for the St. Petersburg governor, told Russian TV that 10 people were killed and 50 wounded in the blast, the Associated Press reported. Russia’s National Anti-Terrorism Committee ordered tightened security at all critical transport facilities. Interfax said local prosecutors had turned over the probe to the federal Investigative Committee.

Anthony Roman, a global risk-management expert and president of Roman & Assoc. in Lynbrook, N.Y., told USA TODAY that based on pictures of the damaged train with the doors blown out and scarring on the walls, the blast seemed to occur inside the car.

“We’re still in the fog of war here,” Roman said.  “You can see considerable damage within the cab itself — structural damage and collateral scarring of the internal frame. That’s what you’d expect to see in a blast.”

Roman’s firm observed Russian security first-hand for transportation during the Sochi Olympics and found it "wholly inadequate." But he said that subways and other train systems are difficult to protect from terrorists because of the large number of people using them.

The investigation of the blast will immediately focus on interviewing witnesses, gathering evidence from the scene and studying shrapnel from the explosive to determine what it was and how it was made, Roman said.

“This might not be the end of it,” Roman added. “The next week or so could be a high-risk period.”

St. Petersburg, a city of about 5 million people on the Baltic Sea in western Russia, was founded by Peter the Great and served as Russia's capital for two centuries. The city remains a cultural center. Tass said the explosion took place near the Sennaya Ploshchad station, or "Hay Square," historically a trading center for hay, straw and wood.