U.S. President Donald Trump spoke with Russian President Vladimir Putin on Monday to offer condolences to the victims of a bomb blast on a St. Petersburg train that either killed or injured more than 50 victims altogether, the White House said in a statement.
"President Trump offered the full support of the United States government in responding to the attack and bringing those responsible to justice," the statement said.
"Trump and President Putin agreed that terrorism must be decisively and quickly defeated," it said.
The main suspect in the blast that killed 14 people and wounded 50 is a Russian citizen originally from mainly Muslim Kyrgyzstan, authorities there said on Tuesday.
The perpetrator had radical Islamist links, Russian media cited law enforcement officials as saying, raising the possibility the attack could have been inspired by Islamic State, which has not struck a major city in Russia before.
There has been no official confirmation or claim of responsibility. The Kyrgyz GKNB security service identified the suspect as Akbarzhon Jalilov, born in the city of Osh in 1995, but provided no other details.
The explosion on Tuesday in the middle of the afternoon occurred when the train was in a tunnel deep underground, amplifying the force of the blast. The carriage door was blown off, and witnesses described seeing injured passengers with bloodied and blackened bodies.
State investigative authorities said fragments of the body of the suspect had been found among the dead, indicating that he was a suicide bomber.
Russian Foreign Minister Sergei Lavrov said it was cynical to say the bombing in St. Petersburg was revenge for Russia's role in Syria. He said the attack showed that Moscow needed to press on with its fight against global terrorism.