An explosion has ripped through a crowded commuter train near Chechnya in Russia, killing at least 36 people.
Investigators found unexploded grenades in the train
The blast derailed the train, full of students and workers, between the towns of Mineralniye Vody and Kislovodsk.
FSB security agency has found the body of a suicide bomber near a bag which may have held the explosives, Russian news agency Itar-Tass reported.
President Vladimir Putin said the attack was a bid "to destabilise the country" on the eve of elections.
Itar-Tass quoted the president as saying "the criminals will get nothing out of this" at a meeting with top security officials.
A train blast in the same area left six people dead in September.
The explosion on Friday happened shortly before 0740 local time (0440GMT).
Russia's emergencies ministry has said that it was the result of a "terrorist act", and law enforcement officials are treating the incident as terrorism and premeditated murder.
Russian Interior Minister Boris Gryzlov vowed to track down the "animals" behind the explosion.
"The ground will burn under their feet. These animals will never be able to feel safe," he told a gathering of war veterans.
Officials say the bomber is thought to have been a woman.
The blast destroyed the train's second carriage.
An FSB spokesman told Itar-Tass: "Next to the body of the suicide bomber a bag was found which, to all appearances, contained the explosive device which went off in the commuter train.
"The type of the explosive device has been identified, its main component was
a plastic-based explosive. The power of the blast was equivalent to between six and 10 kg of TNT. The investigation continues."
At least 15 people were killed at the scene, with more dying of their wounds in hospital. Around 150 people are reported to have been injured; some were treated at the scene while the majority were taken to hospital.
RECENT RUSSIA BLASTS
15 Sept: At least two killed at Russian security HQ in Magas, Ingushetia
3 Sept: At least four killed on Mineralnye Vody train
25 Aug: At least three killed at Krasnodar bus stops
1 Aug: 50 killed at Mozdok hospital
Local officials described pulling bodies from the train, which was thrown onto its side.
Some of the victims were hurled from the train by the force of the blast, the BBC's Sarah Rainsford in Moscow says.
There are about 30 ambulances at the scene, which is littered with shattered glass. An electrical fire is hampering relief efforts, the Associated Press news agency reports.
Senior officials have already left Moscow to investigate.
Russia has recently been rocked by bombings and other attacks, which the government blames on Chechen rebels.
No-one has claimed responsibility for the train explosions.
September's train explosion came on the first day of campaigning for the poll.
State Duma Chairman Gennady Seleznyov also said the attack was planned "in order to trigger panic and instability."
The earlier blast also coincided with a visit by Mr Putin to the southern city of Rostov for a meeting of the country's State Council.
The president has long claimed to have the situation in Chechnya under control.
But with another blast having struck the same train line where people were killed just three months ago, serious questions will be raised about how stringent security measures are, our correspondent says.