A blast in a St Petersburg train carriage on Monday that killed 11 people and wounded 45 was carried out by a suspected suicide bomber with ties to radical Islamists, Russia's Interfax news agency cited a law enforcement source as saying.
Russian President Vladimir Putin, who was in the city when the blast struck, visited the scene of the explosion late on Monday night and laid a bunch of red flowers at a makeshift shrine to the victims.
Witnesses said they saw passengers who were bloodied and burned spilling out of the train, the door of which was buckled by the force of the explosion, and lying on a platform while smoke filled the station.
Russia has experienced bomb attacks carried out by Islamist rebels from Russia's North Caucasus region in the past. The rebellion there has been largely crushed, but Russia's military intervention in Syria has now made it a potential target for attacks by the Islamic State group, security experts say.
There was no immediate claim of responsibility. Officials said they were treating the blast as an act of terrorism, but there was no official confirmation of any link to Islamist radicals.
Earlier, Russian media broadcast closed circuit television footage of a bearded man they said was being sought by police as a suspect. However, Interfax reported that the man had come forward and been eliminated from inquiries.
The news agency, quoting an unidentified law enforcement source, said that human remains examined at the scene suggested that the blast had been carried out by a suicide bomber. It said the police had identified a suspect with links to radical Islamist groups banned in Russia.
If it is confirmed that the bomb was carried out by radical Islamists, the Kremlin is likely to argue the attack underlines the importance of its campaign in Syria, where it is backing President Bashar al-Assad in a fight against Islamist militants.
However, some sections of Russian society could see the metro bombing as proof that Putin's decision to intervene in Syria has again made Russian civilians into targets.
Two years ago, Islamic State said it had brought down a plane carrying Russian tourists home from a Red Sea resort. All 224 people on board the flight were killed.
U.S President Donald Trump offered his condolences to Putin in a phone call late on Monday, the White House said in a statement, offering the support of the U.S. government. The two leaders "agreed that terrorism must be decisively and quickly defeated", the statement said.
China also condemned the attack, with Foreign Minister Wang Yi offering condolences and support in a statement that said "China resolutely opposed all types of terrorism".
Soon after the blast happened at 2:40 p.m. (1140 GMT), ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad station. One helicopter hovered overhead and then landed on a broad avenue to take away a wounded passenger.
"I saw a lot of smoke, a crowd making its way to the escalators, people with blood and other people's insides on their clothes, bloody faces," St Petersburg resident Leonid Chaika told Reuters by phone.
"Many were crying," said Chaika, who was at the station where the blast happened.
The National Anti-Terrorist Committee said an explosive device had been found at another station, hidden in a fire extinguisher, but had been defused.
The blast raised security fears beyond Russian frontiers. France, which has itself suffered a series of attacks, announced additional security measures in Paris.
Video from the scene showed wounded people lying bleeding on a platform, some being treated by emergency services and fellow passengers. Others ran away from the platform amid clouds of smoke, some screaming or holding their hands to their faces.
A huge hole was blasted in the side of a carriage and the door blown off, with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage after the train had pulled into the station.
Russian TV said many had suffered lacerations from glass shards and metal, the force of the explosion amplified by the confines of the carriage and the tunnel.
Anna Sventik, a St Petersburg resident, said she was traveling on a metro train that passed through the same station moments after the blast.
"Our train slowed down a bit, and one woman started having hysterics when she saw the people lying on the platform, blackened, in some places with no clothes, burned," she told Reuters. "It was very scary."
All stations closed
Earlier on Monday, officials had put the death toll from the explosion at 10 people, but the National Anti-Terrorist Committee, a state agency, later said 11 people were killed and 45 were being treated for their wounds in hospital.
Authorities closed all St. Petersburg metro stations. The Moscow metro said it was taking unspecified additional security measures in case of an attack there.
Russia has been on particular alert against Russian-speaking rebels returning from Syria, where they have fought alongside Islamic State, and is wary of any attempts to resume attacks that dogged the country several years ago.
At least 38 people were killed in 2010 when two female suicide bombers detonated bombs on packed Moscow metro trains.
More than 330 people, half of them children, were killed in 2004 when police stormed a school in southern Russia after a hostage taking by Islamist militants. In 2002, 120 hostages were killed when police stormed a Moscow theater to end another hostage-taking.
Putin, as prime minister, launched a 1999 campaign to crush a separatist government in the Muslim southern region of Chechnya, and as president has continued a hard line in suppressing rebellion.