Four people were killed on Monday after a bus ploughed into a pedestrian underpass in the Russian capital Moscow, in what officials said was not a terrorist attack.
Moscow police said passengers and pedestrians were among those killed in Monday's crash.
Police immediately ruled out a possibility of it being an attack, saying that they suspect a mechanical fault or that the driver lost control of the vehicle. Police were questioning the driver.
"The driver of the bus began movement and then changed his trajectory, which resulted in the bus going down into the pedestrian underpass," the statement said.
"The driver has been detained. Police employees are working with him," the statement said.
"According to the driver, the bus was parked and suddenly began to move by itself," said a statement by the Russian Investigative Committee, which probes major crimes.
"Despite his attempts to stop the bus, the breaking system did not work and the bus continued moving," it said.
Investigators said they were questioning witnesses and plan to examine vehicle documentation and order medical examination of the driver.
"Nine people are currently in the city's clinics... four people died on the spot," the Moscow city hall said in a statement on the website.
Footage broadcast on national television showed the bus driving onto the pavement and then down the stairs of the underpass, running over several pedestrians before coming to a stop.
The accident happened around 3 pm (1200 GMT) near Moscow's Slavyansky Boulevard metro station, a prestigious residential area near one of the capital's main avenues.
Sirens were blaring around the metro station, with authorities preparing to pull the bus out of the underpass, an AFP correspondent at the scene said.
The underpass and surrounding area were cordoned off and several black body bags could be seen on the pavement, partially covered with falling snow.