Updated: 8:00 p.m. ET

(Bloomberg) – A subway bombing in St. Petersburg killed 11 people and injured dozens more during a hometown visit by President Vladimir Putin, officials said, renewing fears of terrorism in Russia's biggest cities less than a year before presidential elections.

A homemade device filled with shrapnel detonated inside a train between two central hubs at about 2:40 p.m., the National Anti-Terror Committee said. Footage from the Sennaya Ploshchad station about a mile from the Hermitage Museum showed carriage doors blown open, with bloodied and dazed passengers lying on the underground platform amid billowing smoke.

Officials called the blast, and the planting of a more powerful bomb that was later defused at a nearby station, an act of terrorism, though investigators cautioned that other possible causes can't be ruled out. Police are hunting for two suspects they believe planted the two devices, the Interfax news service reported, citing an unidentified law enforcement official.

“All the signs of a terrorist attack are there,” Viktor Ozerov, head of the security committee in the upper house of parliament, said by phone from Moscow. “The complex of measures against terrorism in the country failed.”

U.S. President Donald Trump, asked about the blast by reporters at the White House, called it a “terrible, terrible thing — happening all over the world.” Kremlin spokesman Dmitry Peskov said later that the bombing was being discussed with the White House through diplomatic channels.

High alert

Officials in St. Petersburg, a city of 5 million people, shut down the entire subway system, forcing thousands of commuters onto the streets and snarling traffic as emergency vehicles struggled to reach the scene. They also ordered security to be enhanced at airports and other major transport nodes, measures that were repeated in the capital Moscow.

In addition to the 11 who died, 45 injured people were being treated in hospitals, a spokesman for Russia's National Antiterrorism Committee said on Rossiya 24 state television.

Putin visited the Federal Security Service's St. Petersburg branch to be briefed on the subway attack by security agencies, Interfax reported, citing Peskov.

Russia's two biggest cities haven't suffered a major attack in more than five years. The Kremlin tightened security after hundreds were killed by terrorist strikes in the early 2000s that were later claimed mostly by Chechen separatists. Since Putin sent forces into Syria in 2015, Islamic State has threatened to strike at Russia, taking responsibility for the downing of a plane carrying Russian tourists from Egypt to St. Petersburg, which left 224 dead.

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Suicide bombers attacked the capital's subway system in coordinated attacks in 2010 that claimed 40 lives. The last major attacks took place in December 2013, just weeks before the Sochi Winter Olympics, when 30 people died in strikes on a train station and a bus in Volgograd.

Islamic State has vowed to take revenge for Putin's bombing in Syria and has claimed responsibility for some attacks in Russia's mainly Muslim North Caucasus region. Russian-speaking jihadists make up the largest foreign contingent of Islamic State fighters in Syria and Iraq, according to the Institute for International Studies in Moscow.

'Barbaric acts'

“These barbaric acts once again show that the terrorists' main goal is to sow fear and uncertainty and cause instability in society,” Boris Gryzlov, a top official in the ruling United Russia party and a former interior minister, said after Monday's attack, according to Tass.

The bomb used in the subway attack appeared to have had a timer, not detonated by a suicide bomber, Interfax reported, citing investigators. It exploded with a force equal to about 300 grams of TNT, just one-fourth of what was used in the device that was disarmed, the news service said.

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Some political analysts said the Kremlin may seek to capitalize on the bombing like it has with previous terrorist attacks, particularly after the biggest anti-Kremlin protests in several years swept major cities on March 26.

“Experience in Russia shows that the terrorist threat can serve as a justification for political tightening, such as banning mass demonstrations,” said Valery Solovei, who teaches at Moscow's main university for diplomats. “But I don't think the security issue will get the authorities very far this time. People are used to living under the risk of terror.”

Russia is due to hold presidential elections in less than a year. Putin, in power since 2000, is widely expected to seek a final, six-year term.

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