BEIJING, May 17 -- The State Council, China's cabinet, dispatched a work team to investigate a fatal bus accident in northwestern China's Shaanxi province that claimed 35 lives on Sunday.

The investigation team was led by Sun Huashan, deputy director of the State Administration of Work Safety, who claimed it was a serious safety liability from preliminary findings.

Illegal transportation practices were the most lethal factor behind the year's worst accident, said Sun.

The tragedy also exposed shoddy management of the traffic company, said Sun, who demanded a thorough safety examination of local vehicles.

Highway safety checks and tightening supervision of stop-overs have also been requested to prevent such accidents from happening again.

A bus carrying 46 people fell into a ravine some 30 meters deep near a forest park in Chunhua county on Friday afternoon.

Twenty-five people died instantly and ten passed away at hospital. The rest are being treated for injuries.

Passengers on the bus were residents from nearby cities, mostly elders, who joined a two-day sight-seeing and shopping tour organized by a Xiangban trading company in Xi'an, the provincial capital. They were on their way home when the accident occurred.

Local police held four people responsible in custody for further investigation.

Also on Sunday, the Supreme People's Procuratorate (SPP) said in a statement that the investigation led by the cabinet also included prosecutors sent from Beijing.

The SPP staff joined local prosecutors at the county, city and province levels to examine clues that might lead to negligence of duty and other occupational crimes behind the accident.

The SPP staff vowed to pursue the liabilities of people involved in the case in accordance with the law, especially those whose activities constituting crimes.