Russian officials were trying to determine who was responsible for an explosion Monday that killed at least 11 people in a subway train in St. Petersburg, the country's second-largest city and the hometown of President Vladimir Putin.
There was no indication that Putin, who was in St. Petersburg for talks with Belarusian President Alexander Lukashenko, was near the underground blast, which the National Anti-Terrorism Committee said also left 45 people hospitalized.
"Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened," Putin said in televised remarks at the start of his talks with Lukashenko, about an hour after the explosion.
Alexander Kurennoi of the General Prosecutor's Office initially called the bombing a terrorist attack, but later told Interfax that it was too early to draw any definitive conclusions.
Authorities also found and deactivated another bomb at another busy station, according to officials.
Russia has seen numerous acts of terrorism, including twin bombings on the Moscow subway system that killed 40 people and wounded at least 100 in 2010.
The high-speed Moscow-to-St. Petersburg train was bombed on Nov. 27, 2009, in an attack that left 26 dead and about 100 injured.
The attacks have consistently been blamed on Chechen separatists, who have sought to establish an independent Islamist republic in the southern Russian region. Chechen rebel leader Doku Umarov claimed responsibility for the attacks on the Moscow metro and the high-speed train.
No group immediately claimed responsibility for Monday's attack, which occurred on a train between Sennaya Square station and Technological Institute station.
Interfax cited an unidentified source who said a suspect in the blast might have left the explosive device in a bag. Russian state television channel Rossiya 24 said police were looking for two possible suspects.
Videos and photos posted by witnesses and survivors showed a wrecked train car and people lying on the floor of a subway station.
In a phone interview, Marianna Vlastyeva, 43, said her train had entered the Technology Institute station when she saw another train with smoke coming out of it.
"People on my train started panicking, running to the escalator, I ran after them," Vlastyeva said. "I noticed several bodies lying next to the train and people carrying more bodies out of the car, all covered in blood. I'm still shaking. I can't believe [terrorist attacks] are happening in my country again. So sad. I still feel like screaming."
The social media site Vkontakte was abuzz with comments about the explosion.
A woman who gave her name as Tatyana and said she was at Sennaya Square station wrote: "Hell, terrible! People [covered] in blood ... in other people's remains ... charred ..... everyone is being driven from the station ... fear !!!!
The explosive device went off at 2:20 p.m., the anti-terrorism organization said. The device was stuffed with pieces of metal, Valery Parfyonov, an emergency health official, said in televised remarks.
The St. Petersburg subway immediately shut down all its stations, and the national anti-terrorism body said security measures would be tightened at all key transport facilities across Russia.
Russian news media reported that security had also been stepped up at St. Petersburg's Pulkovo airport, including enhancement of controls at the airport's entrances and pre-flight screening areas.
Maxim Liksutov, Moscow's deputy mayor, told Interfax that Moscow authorities were tightening security on the subway in the Russian capital.
St. Petersburg, Russia's second-largest city with more than 5 million residents, is the country's most popular tourist destination. The stations involved in the blast are among the subway's busiest.
Many people expressed outrage and conveyed condolences to the victims and their loved ones.
"What horror when innocent people die," wrote Sergey Rykov in a comment on the Russian website Life Novosti.
Makeshift memorials were created at the Sennaya and Technology Institute stations. And some people brought red carnations, roses and candles to the St. Petersburg government office in Moscow, according to Tass Russian news agency.
Authorities in St. Petersburg declared three days of mourning, starting Tuesday, the Russian media reported.
Officials thanked thousands of cab drivers and volunteers who transported stranded commuters free of charge after the subway shut down for several hours.
White House Press Secretary Sean Spicer said President Trump had been briefed on the bombing.
"The United States condemns this reprehensible attack and act of violence," Spicer said.
Special correspondent Mansur Mirovalev reported from Moscow. Simmons reported from Los Angeles. The Associated Press contributed to this report.
MORE WORLD NEWS
3:50 p.m.: This article has been updated throughout with additional reactions from Russian officials and others.
9:10: This article has been updated throughout with Times reporting.
8:38 a.m.: This article was updated with details about those who died after the blast, as well as news reports about security footage depicting a person who could be responsible.
7:27 a.m.: This article was updated with officials saying a second explosive device had been deactivated.
7:10 a.m.: This article was updated with additional details about the explosion, the ongoing Russian terrorism investigation and a quote from President Vladimir Putin.
6:10 a.m.: This article was updated with reports of 50 injuries and a tightening of security on the Moscow subway.
5:52 a.m.: This article was updated with reports of 10 deaths.