A suicide bomber has been blamed by Russian officials for a train blast that killed at least 37 people and injured 177 in the south of the country.
Russia’s Interfax news agency reported that the man was helped by three female terrorists, two of whom jumped off the train after Friday’s blast near the Chechen border.
One woman was in critical condition with life-threatening injuries, and it was unclear what happened to the other woman, according to the Interfax report.
The male suicide bomber, who had grenades strapped to his legs, died in the blast, Interfax reported.
Russia’s security police force, or FSB, said its investigation had turned up the remains of a suicide bomber and a bag or suitcase that contained the explosive materials.
The powerful bomb detonated inside the train’s second carriage and the force of the explosion blew people out of it, according to ministry officials. Video of the scene showed the entire carriage mangled from the blast.
Rescue workers struggled to pull victims from the mounds of shattered glass and other debris, hours after the blast.
Some of the victims died after being thrown from the vehicle, while fears were rising that people were still trapped in the wreckage.
A small fire broke out in the electrical wiring, hampering rescue efforts.
“The train was moving and then there was an explosion with smoke and ash about,” witness Gleb Kovalenko told Rossiya state television. “It happened inside the carriage.”
The Russian prosecutor’s office described the explosion as a terrorist act and premeditated murder, and launched a criminal investigation.
Of the wounded, 127 remain in hospital, with 12 in a critical condition.
Security experts in the nearby region of Ingushetia are also checking a vehicle found that was laden with explosives and believed to be destined for a terror attack, Interfax news agency reported.
The incidents came shortly before parliamentary elections throughout Russia.
CNN’s Moscow Bureau Chief Jill Dougherty said: “The timing is significant because it is just two days away from parliamentary elections in Russia and that always raises the profile of danger in the country.
“There has been concern about terrorism and now we have this attack.”
She added that Chechnya had been pushed off the election agenda recently, with economic issues taking center stage, but the troubled breakaway republic was “back on the front pages and TV screens.”
Interior Minister Boris Gryzlov, head of the party backed by President Vladimir Putin in Sunday’s polls, vowed to track down the perpetrators, saying: “The ground will burn under their feet.”
The train was near Yessentuki, traveling from Kislovodsk to Mineralnye Vody in Russia’s Stavropol region northwest of the Chechen border, when the blast occurred at 8 a.m. (0500 GMT) Friday.
Dozens were injured, Russia’s Ministry for Emergency Situations said.
Because the area has been the site of similar attacks, Russian officials suspect terrorism by Chechen rebels.
It is the second attack on the same stretch of track in three months. In September, an explosion aboard a passenger train near Kislovodsk killed four and wounded dozens.
Security was supposed to have been improved on the line.
Mineralnye Vody, about 890 kilometers (550 miles) south of Moscow, has seen several bus hijackings in the past decade by Chechen rebels demanding the release of jailed comrades.
Two years ago, a stand-off with Chechen hijackers came to an end in the resort town when Russian commandos stormed the bus and rescued more than 40 passengers. One hijacker was killed in the raid.