A bomb blast tore through a subway train deep under Russia's second-lar.
Russian Federation might need foreign help in its investigation of the deadly bombing on an underground train in St Petersburg, Kremlin spokesman Dmitry Peskov said.
Russian police arrested three people Thursday in connection with a suicide bombing in the St. Petersburg subway system earlier this week.
The Investigative Committee said the suspects are residents of the former Soviet Central Asia region like the bomber, 22-year-old Akbarzhon Dzhalilov, a native of Kyrgyzstan.
There was no immediate information tying those arrested with the suspected bomber, identified as 22-year-old Akbardzhon Dhzalilov.
The impoverished, predominantly Muslim countries in Central Asia are seen as fertile ground for Islamic extremists, and thousands of their residents are believed to have joined IS in Syria and Iraq.
Russian President Vladimir Putin on Wednesday commented on the attack during a meeting in Moscow with security officials from former Soviet countries.
Mr Putin thanked Mr Trump for the expression of solidarity, the Kremlin said, adding that the two leaders voiced a shared view that "terrorism is an evil that must be fought jointly".
President Vladimir Putin was in St. Petersburg earlier on the day of the attack.
No terrorist groups have claimed responsibility for the attack.
Russian authorities launched a series of raids across St Petersburg on Wednesday, detaining six people of Central Asian origin believed to have served as recruiters for terrorist groups, including the so-called "Islamic State" (IS) jihadist group.
"Erlan Abyldaev, the Kyrgyz Foreign Minister, confirmed the bombing was a suicide attack but said the perpetrator" s motives were unclear. Later, his parents returned to Kyrgyzstan, while Dzhalilov himself remained in Russian Federation to earn money, Kyrgyz news agency 24.kg quoted their close relatives as saying.
They have also detained several people suspected of being accomplices of the man behind the metro bombing, Russian news agencies have reported.
Investigators scoured the St. Petersburg home of Dzhalilov and reviewed surveillance footage that shows him leaving his home with a bag and a backpack.
A suicide bomber was behind the attack that killed 14 people and left dozens wounded. About 50 others remained hospitalized, some in grave condition. A more powerful bomb was discovered there and defused soon after the explosion on Monday afternoon, on a train that had just departed another station, Sennaya Square.
The decision to keep moving was praised by authorities, who said it helped evacuation efforts and reduced the danger to passengers who would have had to walk along the electrified tracks.
Viktor Khasiyev said he got a call from his father who was in the auto hit by the explosion: "I heard screams, and then he said: 'Son, we got blown up". He worked at a vehicle fix shop and a sushi bar, and stayed in the city when his family returned home.
Security agencies and the Federal Security Service report to the President. And the big fear of the Russian authorities is that some of those people will make their way back and plot attacks here.