At least 14 people were killed and dozens more were injured after an explosion ripped through a St Petersburg metro station on Monday.
Video footage on social media showed terrified passengers attempting to help the wounded. A huge hole was created in the side of one carriage with mangled metal wreckage strewn around the platform.
"People were bleeding, their hair burned," one eyewitness told LifeNews.
"We were told to move to the exit, because the movement stopped.
"People just fled. 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."
Where did the explosion occur?
The St Petersburg metro press office said the blast was caused by a shrapnel-loaded explosive device, adding that all stations had been shut down after the incident. The city's airport was also closed.
According to local media reports, the explosion occurred near Sadovaya and Sennaya Ploshchad stations.
Initial reports said there were two explosions, but a Russian emergency services source later disputed this in an interview with local media.
The home-made explosive device was thought to be equivalent to 200 grams of TNT and was placed on the carriage.
Who is the suspect?
Authorities in Kyrgyzstan they believed the attack had been the work of a suicide bomber, and said the perpetrator was suspected to be a 22 year old Kyrgyzstan-born man who obtained Russian citizenship named Akbarzhon Jalilov.
A bearded man who appeared on CCTV footage and who Russian television stations initially claimed was the suspected attacker turned himself into the police, saying he was innocent, after seeing himself on television. Interfax news agency reported that the man has since been eliminated from inquiries.
Other CCTV images provided by police to Russian television channels and redistributed by Reuters, which purport to show Jalilov, show a young man in a red parka and carrying a rucksack walking through a metro station.
Eyewitness: 'A thundering clap, then smoke, then a strong smell'
"We were riding in the neighboring car, and at that time it was very crowded: all the seats were taken and many were standing," said one eyewitness.
"The explosion went off between stations. There was a thundering clap, followed by a strong smell smell and smoke. We all moved to the opposite end of the wagon, people jammed together and two women passed out.
"This all was happening while the train was still moving, it didn't stop. Everyone got out at Tenologicheskom Institute station. There, we saw that the neighboring wagon was shattered, the windows blown out, no light, blood."
Explosion was act of terror
The Russian authorities have confirmed that the explosion was an act of terror.
It comes after Vladimir Putin, the Russian president, visited the site of the explosion and offered his condolences to the victims, adding that investigators viewed a "possible terror attack" as one of several reasons behind the incident.
Are terror attacks common in Russia?
Russia has been the target of attacks by Chechen militants in past years. Chechen rebel leaders have frequently threatened further attacks.
Monday’s attack was the deadliest outside the Caucasus since two suicide bombers killed 32 people in the southern city of Volgograd in December 2013.
Suicide bombers also targeted the Moscow Metro in 2010, when two female suicide bombers killed at least 38 people, and Moscow's Domodedovo international airport in 2011.
Over 330 people, half of them children, were killed in 2004 when police stormed a school in southern Russia after a hostage taking by Islamist militants.
In 2002, 120 hostages were killed when police stormed a Moscow theatre to end another hostage taking.