Four people have been killed in Moscow after a passenger bus swerved off course and slammed into a busy pedestrian underpass on Christmas Day.
Photos taken at the scene show the bus on the steps leading into the underground passageway. "Police employees are working with him".
According to the source, there are two main reasons for the crash: either the bus or another auto cut it off or the brakes on the bus failed.
It was not immediately clear what caused the crash.
Police sources told TASS the investigators are considering two versions of the accident - the driver lost control of the bus or else the latter was out of service.
It has been reported that five people were killed as a result of the incident, the causes of which are yet to be established.
"According to the driver, the bus, while being at a bus stop, suddenly began to move independently despite the driver's attempts to stop it because the braking system did not work", the Russian Investigative Committee said in a separate statement.
Local reports suggest the driver was arrested on suspicion of unsafe driving.
The incident occurred on one of the Russian capital's busiest roads near the Slavyansky Boulevard metro station.
The driver was detained and the authorities instituted a criminal case citing the services falling short of security requirements and encroachments on the safety regulations for public transport.
Sirens were blaring around the metro station, as authorities prepared to pull the bus out of the underpass, an AFP correspondent at the scene said.
Sobyanin also visited the scene, telling journalists he had ordered a full inspection of the entire Moscow bus fleet.