Four people were killed and 15 injured after the bus they were traveling on rolled around noon Wednesday on a highway in Shaanxi Province, according to local authorities.

Two people were killed instantly when the bus overturned after one of its tires went flat, and two others died in hospital after emergency treatment, a spokesman with the county government of Qianxian said.

Fifteen people were injured slightly and were being treated at two local hospitals, the spokesman said.

The bus with 21 people onboard was traveling from Qianxian County to Xianyang City, he said.

Local authorities are investigating the cause of the accident, the spokesman added.