One person was believed to be behind the deadly attack in the subway in St. Petersburg, Russia, planting two bombs, one of which exploded, state media reported Monday.
At least 11 people were killed and some 45 others were wounded. Police initially believed a suicide bomber planted the device that exploded on a train, while a second person planted a separate device at a nearby station -- but investigators later said it appeared to be the work of one man, the Interfax news agency reported. Crews disabled the second device before it could explode.
Both bombs were filled with shrapnel, according to Sky News. The unexploded device was rigged with up to 2.2 pounds of explosives, Interfax added.
Russian President Vladimir Putin said investigators were looking into all possible causes. President Trump called it "absolutely a terrible thing," adding, "it's happening all over the world."
Late Monday, the White House said Trump called Putin to express his condolences for the bombing and to offer the full support of the United States "in responding to the attack and bringing those responsible to justice."
"We will look at all possible causes, terrorism as well as common crime."
The 2:20 p.m. explosion rocked the train between the Technology Institute station and the Sennaya Square station, Russia's National Anti-Terrorist Committee said. Photos and video from one station appeared to show wounded victims on the smoke-filled platform, and a train car with a door blown out. Frantic commuters reached out through the doors and windows, shouting, "Call an ambulance!"
Nobody immediately claimed responsibility for the blast. The National Anti-Terrorism Committee reported late Monday an 11th person had died.
"People were bleeding, their hair burned," a witness told Russia's Life News. "My girlfriend was in the next car that exploded. She said that he began to shake. When she came out, she saw that people were mutilated."
Russian media, The Associated Press and other news agencies sent out a photo of a bearded man dressed in black, initially claiming he was wanted in connection with the blast. Interfax later reported that the man went to police to claim he was innocent.
Trains and train stations have been common targets for terrorist attacks in Russia and throughout much of Europe, analysts point out.
Double suicide bombings in the Moscow subway in March 2010 killed 40 people and wounded more than 100 people. Chechen rebel leader Doku Umarov claimed responsibility for that attack by two female suicide bombers, warning Russian leaders that "the war is coming to their cities."
A high-speed Moscow-to-St. Petersburg train was bombed on Nov. 27, 2009 in an attack that left 26 dead and some 100 injured. Umarov's group also said he ordered this attack.
Crews closed all subway stations in St. Petersburg Monday and evacuated passengers, administration officials said. Ambulances and other medical teams rushed to the scene.
The National Anti-Terrorism Committee vowed to tighten security at all of the country's critical transportation centers.
"The safety and security of U.S. citizens overseas is one of our highest priorities. The U.S. Consulate in St. Petersburg is monitoring the situation closely," a State Dept. spokesperson told Fox News.
U.S. Ambassador to the United Nations Nikki Haley tweeted a statement, part of which read: "We would like to extend our condolences to the people of Russia. We stand with you on defeating these extremist groups who continue to cause senseless harm against innocent people."
St. Petersburg, Russia's second-largest city with more than 5 million residents, is the country's most popular tourist destination. The two stations that were the site of the blast are some of the subway's busiest.
"The causes are not clear, it's too early. We will look at all possible causes, terrorism as well as common crime," Putin responded. "Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened."
The Russian president offered condolences to the families of the victims. He was visiting the city -- his hometown -- and held talks Monday with the president of Belarus, Alexander Lukashenko.
The Sennaya Ploschad station opened in 1963, the BBC reported.
German Foreign Minister Sigmar Gabriel said he learned of the explosion "with deep sorrow." He added that Germany's thoughts were "with our friends in Russia, the victims and their families in this dark hour." French Foreign Minister Jean-Marc Ayrault said his country would "stand by all those who suffer."
The Associated Press contributed to this report.