MOSCOW, July 16 -- A bus ran into a ditch and overturned in eastern Russia's Mari El Republic early Sunday morning, injuring all 29 passengers, said the regional ministry of internal affairs.

Six of the injured suffered fractures of moderate severity and the rest sustained bruises and abrasions. The authorities said they have received medical treatment.

The driver may have fallen into sleep at the wheel and lost control of the bus when the accident happened, said the ministry.

Investigators are working on the site to investigate the cause of the accident.