LANZHOU, May 5 (Xinhua) -- An agricultural tricycle rolled over into a roadside trench in northwest China's Gansu Province Sunday, leaving four passengers dead and the rest two aboard the vehicle injured, local authorities said.

The accident occurred at about 7 a.m. as the tricycle fell into a five-meter-deep trench beside a country road in the Xigu district of Lanzhou, the capital city of Gansu, the district's work safety authorities said in a statement.

The two injured are under medical treatment at a hospital.

All the six aboard the tricycle are identified as migrant workers en route to a construction site.

An initial investigation shows that the brake failure is to blame for the fatal accident. Police are further investigating the case.

Agricultural tricycles are banned from carrying passengers in China.