A powerful bomb ripped through a passenger bus Wednesday morning in a central Russian city, killing eight people and injuring more than 60, officials said.

Investigators were trying to determine whether the explosive device had been carried by a passenger or planted somewhere inside or beneath the bus in the Volga River city of Togliatti. Yuri Rozhin, the head of a local branch of Russia's Federal Security Service, the main KGB successor agency, said in a televised statement that the bomb could have been detonated by a homicide attacker.

With nearly a month remaining before parliamentary elections, the blast raised fears of violence similar to that which has occurred before past elections.

"Due to (the blast's) character, its consequences, the main version being considered is a terrorist attack," Rozhin said.

But officials said that other versions were also being considered.

"A terror attack is a likely theory, but not the main one," Alexander Konovalov, Putin's envoy to the Volga River region, said, according to the Interfax news agency. He said that the investigators were also looking at careless handling of explosives or a turf battle between criminal clans as possible explanations.

Togliatti is headquarters to Russia's largest carmaker, AvtoVAZ, and has a reputation for gang violence as varying groups have competed for control over the lucrative state-owned factory. A factory spokesman could not say whether there were factory workers among the victims.

Images on Russian television showed a long green bus, its windows blown out and its roof partially detached from the force of the explosion. Paramedics attended to people with bloody faces and legs.

Valery Matkovsky, a local emergency official, said that eight people had died and 53 suffered burns and shrapnel wounds. Russian media said that a child was among the dead.

Konovalov, Putin's envoy, said later that 63 people were injured.

The explosion, which shattered windows on nearby residential buildings, occurred near a bus stop in the city center as people were going to work. A group of college students had left the bus at the stop just seconds before the blast, and about 20 students were among the injured, NTV reported.

Vadim Blagodarny, a 20-year-old photographer, said people walked around stunned and in shock in the minutes after the blast, as investigators picked through the carnage.

"If it had gone off just a minute earlier, it would have been much, much worse," he said.

Security at the scene was heavy, and some photographers were either detained or had their equipment confiscated.

Some Russian politicians immediately speculated that the blast was an attempt to sow social unrest. Earlier this year, the head of the country's main security agency warned of the potential for pre-election violence, and said police agencies would bolster security and surveillance nationwide before the Dec. 2 vote.

In 1999, just three months before national elections, there were explosions in several residential buildings in Moscow and other Russian towns, killing hundreds. Opposition activists and Kremlin critics said the government used the blasts to justify sending federal troops back into violence-wracked Chechnya, launching the second war in a decade in the region.