A bus fire that killed 11 young children and two adults in Shandong province last month is thought to have been started by the driver, who was among the dead, police said on Friday in Weihai, Shandong province.

The bus driver, Cong Weizi, is suspected of starting the fire to vent his anger over his overtime and night shift subsidies being suspended, Wang Jincheng, deputy head of the Shandong Provincial Public Security Bureau, said at a news conference on Friday in Weihai.

Cong is thought to have bought gasoline from a gas station and set it on fire in the bus, Wang said.

"The fire was started on the floor behind the driver's seat. A cap to a lighter was found among the combustion residues near the driver's seat. Gasoline residues were found at multiple spots on the bus," he said.

"Electrical faults and a traffic accident have been ruled out as the cause of the fire," said Wang, leader of the investigation team that handled the case.

The diesel-powered bus with 13 people on board was involved in a rear-end crash inside a tunnel shortly before the fire started at around 9 am on May 9. But after investigating, police said evidence points to arson.

All on board were killed, including a female teacher who died three days after the fire. Of the 11 children killed in the fire, five were from the Republic of Korea and six were Chinese nationals.

The children, aged 3 to 6, were all students at a kindergarten that is part of the Weihai Zhongshi International School, which was set up in 2006 primarily to accommodate children of ROK businesspeople in Weihai.

The Weihai Zhongshi International School rented the bus from the tourism and rental branch of the Weihai Public Transport Group to take the children to school.

The Chinese leadership has expressed great concern about the incident. The Foreign Ministry issued instructions on dealing with the aftermath on May 10, the day after the fatal fire.

Foreign Ministry spokeswoman Hua Chunying on Friday again expressed condolences to the families of the deceased. She said China will continue to spare no effort in handling the incident.

The Ministry of Public Security sent a team of experts to Weihai to guide the investigation.

Weihai, a port city across the Yellow Sea from the Korean Peninsula, has close business ties with the ROK. Companies from the ROK, including Samsung Electronics, have business operations in the city.

zhaoruixue@chinadaily.com.cn