A bus swerved through traffic, veered off a bridge and plunged into a river on Friday in St. Petersburg, Russia's second-largest city, killing seven people, officials said.

The Investigative Committee, Russia's top criminal investigations body, reported the death toll. It did not state how many others were injured, but the emergencies ministry earlier said that six people removed from the bus were in critical or serious condition.

Russian news reports said there were 15 to 20 people on the city bus when it broke through a barrier and plunged into the Moika River in central St. Petersburg. Six of those who were onboard climbed out of the water on their own.

A surveillance video released by the Russian media showed the bus driving fast, making a sharp turn onto the bridge, swerving across lanes and colliding with another vehicle before breaking through the barrier and falling into the water.

One witness described bystanders jumping into the water to try to help rescue passengers.

"They just dove in with their clothes on and helped," she said in Russian.

Authorities in St. Petersburg said that the owner of the bus had been fined 23 times for various violations. Private companies run most of the city's bus services.

Another witness said it looked like "either he was steering erratically or the brakes failed."

The bus driver was detained by police. His wife was quoted by Russian media as saying that managers forced him to work a morning shift after working for 20 hours the previous day and getting virtually no rest.

Authorities opened a criminal investigation into alleged traffic violations and unsafe travel services.

In:
• Russia
• St. Petersburg