ST. PETERSBURG, Russia — A large explosion blew a hole in the side of a train as it traveled between two subway stations in St. Petersburg on Monday, sending panicked commuters diving for cover, in what authorities were investigating as a possible terrorist attack.
By Tuesday, the death toll stood at 14.
The blast rocked the heart of Russia’s second largest city and prompted calls for increased security measures in the country’s public spaces. At least two suspects were being sought, officials said, but they gave no immediate details on their identities or suspected affiliation.
President Vladimir Putin was holding talks in St. Petersburg, his hometown, with Belarusan leader Alexander Lukashenko at the time of the explosion.
Even as authorities investigated the scene of the blast, which injured nearly 40 people, a separate shrapnel-filled explosive device was found and disarmed at another subway station.
The discovery further focused the inquiry on a possible coordinated plot, and Russia’s Investigative Committee said it was looking into the incident as a possible terrorist attack by Islamic militants.
There was no immediate claim of responsibility, but speculation focused on extremists from southern Russia who left Chechnya and elsewhere in the Caucasus and joined the Islamic State militant group.
A surveillance photo showed a man with a long beard, wearing black clothing and a kufi cap — all indicative of observers of strict Sunni Islam in the restive North Caucasus of southern Russia.
Security forces fanned out on extra patrols as police helicopters crisscrossed overhead in one of the city’s most celebrated, and tourist-visited, neighborhoods. The area around the Sennaya Ploshchad station is near some of the most famous sights of St. Petersburg, and was the setting of Fyodor Dostoevsky’s novel ‘‘Crime and Punishment.’’
In a televised statement less than an hour after the explosion, Putin said he had spoken with the leaders of the Federal Security Service and with law enforcement officials, whom he said would “do everything to find out the causes of what had happened.”
Speaking from the Constantine Palace in the Strelna district of St. Petersburg, about 10 miles west of the blast, Putin said, ‘‘Naturally, we always probe all theories, both domestic and criminal ones, primarily actions of a terrorist nature.’’
Shortly after the blast, the entire St. Petersburg subway system was shut as a precaution, and security was boosted around the city.
Outside the city’s main train station, more than 100 stranded passengers lined up for rides — not just from city transport but also from friendly drivers in a ride-sharing program.
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 2:40 p.m.
The operator was able to get the train to the next station, where authorities reached the victims, Przhezdomsky said on state television. Amateur video shared on social media showed a large, jagged blast hole in the side of one car.
The St. Petersburg office of the Federal Security Service said an ‘‘unexploded device’’ filled with shrapnel was found at the Ploshchad Vosstaniya metro station. The device was defused by a bomb squad. No one was injured in that incident.
The station is, like Sennaya Ploshchad, is one of the central interchanges.
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 six people remained in serious condition, raising the possibility that the death toll could rise. Authorities late Monday said an 11th person had died.
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.
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.
Islamist militants from the North Caucasus have been blamed in more than a dozen major terrorist attacks in Russia since the country fought two civil wars in Chechnya. Russia still faces a simmering insurgency in the neighboring Dagestan province, and in March, six Russian soldiers and six militants were killed in a shootout in Chechnya.
In sending the military to Syria in September 2015, Putin said his aim was to battle militants on their own turf before they could attack Russia.
The city of St. Petersburg announced three days of mourning beginning Tuesday. In Washington, President Trump called the incident a ‘‘terrible thing.’’
All St. Petersburg and Moscow subway stations have metal detectors and baggage scanners, though officers only sporadically put larger packages through a baggage scanner. They permit items such as concealed cameras to pass through.
