An estimated 42 people were killed 160 were injured Friday morning when a bomb ripped through a packed commuter train near Chechnya.

The head of the Federal Security Service, Nikolai Patrushev, is blaming the bombing on four attackers, including three women, according to the Interfax news agency.

Earlier, CTV's Ellen Pinchuk said there were reports that one of the bombers is still alive and in hospital.

"And that is the what the FSB, the security services, are looking into now as the suspects for the people who were committing this act."

The bomb went off inside the train's second car at around 8 a.m. as it was travelling between the cities of Mineralnye Vody and Essentuki, according to Maj. Gen. Nikolai Lityuk of the Emergency Situations Ministry.

It was packed with mostly students and workers, and appeared calculated to kill and injure as many people as possible.

As many as 160 people have been admitted to hospitals in the region, Lityuk said.

The blast was so strong that many of the dead were thrown from the train, and the car flipped onto its side. Bomb experts entered the car to set off undetonated explosives, sending off three booms, Russian state television reported.

No one has claimed responsibility for the blast.

Russia has been rocked by a series of explosions, which it normally blames on Chechen rebels.

The rebel Chechen government issued a statement to the media denying any role in the bombing. "We therefore condemn any acts of violence that directly or indirectly target the civilian population anywhere in the world."

Vladimir Rudyak, a spokesman for the prosecutor's office in the region, said authorities were investigating the blast as a terror attack.

Russia President Vladimir Putin said this latest bombing was an attempt to destabilize Russia ahead of elections, to be held on Sunday.

"The criminals will get nothing out of this," the Itar-Tass news agency quoted him as saying at a meeting with top security officials.

A train blast on the same train line in September killed six people, and came on the first day of the campaign.

Pinchuk said there were reports earlier in the week that female suicide bombers, called black widows, were coming into Russia from Chechnya. They were reportedly going to coincide some kind of terrorist act with the election to "show things are not stable in the region" and to remind the electorate that the Chechen issue still remains an issue.

Russian forces withdrew from the southern Russian republic after a brutal 1994-1996 war. They returned in 1999 after Chechnya-based militants invaded a neighbouring region.