CHONGQING -- Slippery roads may have contributed to a bus accident that has left 24 people confirmed dead and seven others injured on Monday morning in southwestern municipality of Chongqing.

The bus drove through the guardrail of a highway overpass and plunged more than 20 meters to the ground.

Five of the injured are in a critical condition, according to the public security department of Chongqing.

Police are trying to determine the exact number of passengers who were on the bus.

The accident happened at around 8:25 a.m. as the bus was traveling from the Yubei District to Beibei District.

All the injured have been sent to the NO. 9 People's Hospital of Chongqing and the traditional Chinese medicine hospital of Beibei District.

A rescuer who declined to give his name told Xinhua that 22 people died at the scene and two others died on the way to hospital.

While police are continuing their investigation they say overnight rain may have caused the road to become slippery, causing the driver to lose control of the bus.

Government leaders, including Zhou Mubing, Chongqing's vice mayor, have been supervising the rescue operation.

Last October a bus drove off the elevated on-ramp of a Chongqing bridge killing 30 people and injuring 20.