XI'AN, July 21 -- Four people died and 27 were injured when a bus overturned Tuesday in northwest China's Shaanxi Province, local authorities said.

The bus, en route from the Uxin Banner of Inner Mongolia Autonomous Region to Xi'an, capital of Shaanxi Province, overturned on an interprovincial expressway in Jingbian County at 5:23 p.m., said a spokesman with the county fire brigade.

He said the fire brigade sent dozens of firemen to the rescue.

Of the 31 people on the bus, two were killed at the scene and two others died later in hospital, the spokesman said.

The injured were all being treated at local hospitals in Shaanxi.

A preliminary investigation suggested the bus had overturned due to a brake failure, as the roads were slippery after a sudden downpour Tuesday afternoon.

Investigation was continuing.