Kyrgyzstan's state security service says the main suspect in the St. Petersburg subway bombing is a Kyrgyz-born Russian citizen.
A spokesman for the State Committee for National Security (UKMK), Rakhat Sulaimanov, on April 4 identified the suspect as Akbarjon Jalilov and said he was born in the Osh region in southern Kyrgyzstan in 1995.
A bomb blast ripped through a subway car in St. Petersburg on April 3, killing at least 11 people and injuring 51 others.
No group has claimed responsibility for the blast, which occurred when President Vladimir Putin was in his home city.
A deputy chairman of Kazakhstan's Committee for National Security (KNB), Nurgali Bilisbekov, told a government meeting that a Russian citizen of Central Asian origin -- but not from Kazakhstan -- is suspected of being behind the blast.
Interfax cited an unidentified law-enforcement official as saying the suspect is believed to have carried an explosive device onto the train in a backpack.
Meanwhile, U.S. President Donald Trump telephoned his Russian counterpart, Vladimir Putin, and offered "full support" from the United States after the blast.
A White House statement late on April 3 said Trump promised "the full support of the United States government in responding to the attack and bringing those responsible to justice."
"Trump and President Putin agreed that terrorism must be decisively and quickly defeated," said the statement, which echoed an earlier account from Kremlin spokesman Dmitry Peskov.
Peskov said Trump had "extended deep condolences" to the families affected by the blast.
WATCH: Multiple Casualties In St. Petersburg Metro Blast
The National Antiterrorism Committee (NAK) said it was looking for the "perpetrators and organizers of the terror attack."
Russia’s health minister, Veronika Skvortsova, said early on April 4 that four of the injured were in critical condition.
The blast occurred midafternoon on April 3 as the train was between stations in the city center.
About two hours after the blast, a homemade explosive device was discovered in another subway station and defused by bomb experts without incident, NAK spokesman Andrei Przhezdomsky told state television.
Russian news media said police were searching for a man recorded on surveillance cameras.
Many of those injured in the attack suffered shrapnel wounds, according to news reports.
Law-enforcement agencies confirmed the device was loaded with shrapnel, and the Interfax news agency said it contained up to 1 kilogram of explosives.
Footage and photos posted on social media showed smoke choking a subway station and dead or injured people lying on a platform next to a damaged subway car.
Images also showed a subway car at a station with a door blown off and the interior badly mangled.
WATCH: Chaos Inside St. Petersburg Metro Following Blast
Natalya Kirillova said she was seated near the end of the subway car that was directly attached to the car where the blast took place. It seemed, she said, that the explosive device may have been placed on the platform connecting the two subway cars.
She said she had just looked at her cell phone, fearing she was going to be late to a 3 p.m. meeting.
"At that moment, it hit me. A deafening explosion. I was seated next to an iron beam, and I think that’s what saved me," she told Current Time TV. "Everyone fell to the right, but not onto the floor, onto their seats."
"There were a lot of women and young children in the car. A grandmother and her child were across from me. She was lucky, though. They just fell down, but weren’t injured. I was totally deafened," she said.
Kirillova said that, after the explosion, the subway continued onto the next station where she and other passengers had to climb through the windows because the doors were broken. After helping the grandmother and child, she turned around and saw a "huge number of people lying down."
"Bodies. It was awful. When we got out [of the subway car], they were pushing and pulling several people out covered in blood," she said. "I saw one woman who had a huge, huge wound on her face."
Putin, who was visiting his hometown of St. Petersburg for a meeting with Belarusian President Alyaksandr Lukashenka, said he had been briefed by security officials on the incident.
Speaking alongside Lukashenka, Putin offered his "most sincere condolences to the loved ones of the victims and the wounded."
"Law-enforcement agencies and the special services are working and will do everything to establish the reasons and the full extent of what has happened," he said.
A somber-looking Putin later brought flowers to the subway station. He walked away to his car without making a statement.
The Antiterrorism Committee said the blast tore through a subway train between the Tekhnologichesky Institut and Sennaya Ploshchad stations in central St. Petersburg at around 2:40 p.m. local time.
Sennaya Ploshchad is one stop away from a main subway transfer point in the heart of the city.
The St. Petersburg metro closed all stations in its network but opened some lines several hours later.
Western governments expressed condolences and solidarity in the aftermath of the attack.
Before his reported call to Putin, Trump described the incident as a "terrible thing."
"Happening all over the world, absolutely a terrible thing," he said during an event at the White House.
The UN Security Council strongly condemned the "barbaric and cowardly terrorist attack."
The 15-member council called for the perpetrators of the attack to face justice.
WATCH: Putin Reacts To St. Petersburg Blast
U.S. Ambassador to the UN Nikki Haley said separately that the images from St. Petersburg in the aftermath of the attack were "heartbreaking."
"You can be sure the United States will stand with Russia on defeating these extremists who continue to senselessly harm innocent people," Haley said in a statement.
The blast brought back grim memories of previous bomb attacks on the metro in Moscow and on trains and buses elsewhere in Russia.
Suicide bombers have struck several times in Russian cities in the past two decades, with insurgents based in Chechnya or other parts of Russia's North Caucasus often blamed or claiming responsibility.
The last fatal attack on a subway system in Russia occurred in Moscow in March 2010, when explosions at two stations killed at least 33 people. There had been no major attacks in St. Petersburg, Russia's second-largest city.