An apparent suicide bombing has killed at least 40 people and injured some 170 on a crowded commuter train in southern Russia, close to the Chechen region.
Investigators found unexploded grenades in the train
The blast struck the train just outside the spa town of Yessentuki during the morning rush hour and many of the victims are said to be young students.
A Russian minister said the attack bore the hallmarks of Chechen rebels.
President Vladimir Putin described it as a bid to destabilise the country days before its parliamentary election.
Justice Minister Yuri Chaika suggested evidence at the scene pointed to "Chechen terrorism" as a possible theory for the blast.
The director of the Federal Security Service (FSB), Nikolai Patrushev, said a male suicide attacker and three women accomplices appeared to have carried out the attack.
Two of the female attackers leapt from the train seconds before the explosion while the third woman was, he added, seriously injured and unlikely to survive.
Mr Patrushev said hand grenades attached to the legs of the male suspect indicated he had been a suicide attacker.
No claims of responsibility for the blast were reported in the immediate aftermath, but a string of similar attacks in recent years have been blamed on Chechen separatist rebels.
The Stavropol area, where the blast occurred, has declared Monday a day of mourning.
The explosion ripped through the second carriage from the front shortly before 0740 local time (0440 GMT) on Friday, 400 metres (yards) outside Yessentuki.
Investigators said a bomb appeared to have been left under a seat and a railway official quoted them as saying it had had the explosive force of 30 kilos (66 pounds) of TNT.
Many of those hit by the blast were students as young as 19 from the town of Kislovodsk who were on their way to study in Pyatigorsk, Russia's Itar-Tass news agency reports.
An incomplete list of the dead published by the authorities in Stavropol suggested the ages of the victims ranged from 16 to 68.
Such was the power of the blast that it tore the carriage in two, knocking it on its side and sparking a fire.
At least 15 people were killed at the scene, with more dying of their wounds in hospital.
Three hours after the explosion, rescuers were still working at the scene, managing to extract one passenger alive but badly injured.
Mr Putin described the attack as "international terrorism" and said it was a clear attempt to destabilise the situation before Sunday's State Duma election.
RECENT BLASTS IN RUSSIA
15 Sept: At least two killed at Russian security HQ in Magas, Ingushetia
3 Sept: At least four killed on Mineralniye Vody train
25 Aug: At least three killed at Krasnodar bus stops
1 Aug: 50 killed at Mozdok hospital
5 July Chechen suicide bombers kill at least 14 at a rock concert near Moscow
"The crime perpetrated this morning bears witness to the fact that international terrorism, which has now thrown down a challenge to very many countries throughout the world, remains a serious threat to our country too."
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.
September's train explosion came on the first day of campaigning for the poll and 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, the BBC's Sarah Rainsford reports from Moscow.