Authorities in eastern China's Shandong Province said on Friday they suspect a fatal bus was started deliberately by the driver, Xinhua reported.
11 children, aged between three and six, and two adults were killed in the blaze inside the Taojiakuang Tunnel in Weihai on May 9.
Of the 11 children, five were South Korean nationals and six were Chinese. They were all students at an international kindergarten in Weihai, according to local officials.
Electrical fault and traffic accident were ruled out as the causes of the fire, police said.
Police said the driver's overtime pay and night shift allowance had been suspended, causing him to buy gasoline and start the fire in anger.