MOSCOW A large explosion blew a hole in the side of a train as it traveled between two central St. Petersburg metro stations Monday, killing at least 10 people and sending panicked commuters running for cover. Anti-terrorism units were immediately brought into the investigation.

Even as authorities probed the scene of the blast – which left more than 30 people injured – a separate shrapnel-filled explosive device was found and disarmed at another subway station.

The discovery further raised suspicions of a possible coordinated attack plot in Russia’s second-largest city, where security forces fanned out on extra patrols as police helicopters crisscrossed overhead.

Shortly after the blast, the entire St. Petersburg subway system was shut down as a precaution, and security was boosted around the city, where Russian President Vladimir Putin was holding talks with Belarusan leader Alexander Lukashenko.

Andrei Przhezdomsky, spokesman for the National Anti-Terrorism Committee, said the blast was caused by “an unidentified explosive device” in one of the cars as the train traveled from Sennaya Ploshchad station, one of the main interchanges of the city’s subway system, at about 2:30 p.m.

The train was able to make it to the next station, where authorities were able to reach the victims, Przhezdomsky said in remarks broadcast on state television. Amateur video shared on social media showed a large, jagged blast hole in the side of one car.

Authorities have not given extensive reports on the direction of the investigation, but counterterrorism experts and others were part of the probe. One detail emerged: officials said the blast was not a suicide attack.

A statement by the St. Petersburg office of the Federal Security Service said an “unexploded device” filled with shrapnel was found at the Ploshchad Vosstaniya metro station, one of four stations in central St. Petersburg.

“The device was defused by a bomb squad. No one was injured,” the statement added.

Russia’s health minister, Veronika Skvortsova, said seven people died at the scene, one died en route to the hospital and two more while undergoing treatment. She said that six people remained in serious condition, raising the possibility that the death toll could rise.

“Law enforcement agencies and the special services are working on it, and they will do everything to find out the causes of the incident and make a full assessment of what happened,” Putin said in televised remarks at the meeting with Lukashenko. “Naturally, we always probe all theories, both domestic and criminal ones, primarily actions of a terrorist nature.”

Viktor Ozerov, a member of the defense and security committee of the upper house of the Russian parliament, told the Interfax news agency that the attack had “all the characteristics of a terrorist attack.” Other legislators called for increased security measures.

The area around the Sennaya Ploshchad station is famous as the setting of Fyodor Dostoevsky’s novel “Crime and Punishment.”

The U.S. Embassy joined other countries in expressing condolences. The city of St. Petersburg announced three days of mourning beginning Tuesday.

All St. Petersburg and Moscow subway stations have metal detectors and baggage scanners, though staff are lax about what and who gets searched.

Officers only sporadically put larger packages through a baggage scanner and permit items such as concealed cameras and tripods to pass through without additional checks.

The state-run news channel Rossiya-24 reported that authorities have video of a possible suspect entering the Sennaya Ploshchad station. Social media were circulating what some Russian agencies called a screen grab of the picture, but it was unclear whether the image was an official one.

Russia has been the site of more than a dozen major terrorist attacks that authorities have blamed on Islamic militants in the North Caucasus region. There, Russia fought two civil wars in Chechnya, and faces a simmering insurgency in the neighboring Dagestan province. In March, six Russian soldiers and six militants were killed in a shootout in Chechnya.