Police in Xiamen reportedly found evidence of gasoline even though the bus uses diesel as fuel.
"According to on-site checks and findings by relevant experts and the local police, the bus’s tires and fuel tank were in complete form when the fire took place," Xinhua reported, citing a statement from the local government of Xiamen.
"Therefore, it can be ruled out that it’s a safety accident. Initial findings conclude that it is a serious criminal case."
The Bus Rapid Transit (BRT) vehicle erupted into flames around 6:30 p.m. local time. About 33 people were also injured in the fire.
One slightly injured passenger told Xinhua that she smelled gasoline and saw another passenger smashing the window in an attempt to escape the bus.
Minister of Public Security Guo Shengkun has arrived in Xiamen to lead a State Council team overseeing the investigation.
President Xi Jinping, who is currently meeting with US President Barack Obama at a summit in California, has warned against pursing development at the cost of people's lives.
"Major accidents have occurred one after another and incurred heavy casualties and property losses. Great attention must be aroused," Xi ordered, according to a statement released by the Central Committee of the Communist Party of China late Friday evening.
China's public system has expanded rapidly over the past decade, prompting concerns that safety is being sacrificed in the rush to provide more modern services to its citizens.
In 2011, a train accident near the city of Wenzhou killed 40 people. Later that same year, a collision on a new subway line in Shanghai injured hundreds of people.
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