An initial investigation showed the bus fire was suspected to be a "serious criminal case", they said.
The investigation by experts and police showed the tyres and oil tank of the bus remained intact while most of its other parts were gutted.
The combustion improver was gasoline while the bus was equipped with diesel engine rising suspicion that it was not an accident, state-run CCTV reported.
A Bus Rapid Transit (BRTS) bus caught fire in the city of Xiamen on elevated highway, in one of the deadliest road accidents in the country recently.
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The fire broke out when the bus was 500 meters away from the Jinshan bus stop, an official said. Several survivors said they smelt gasoline before they noticed the fire.
A government official said the case was initially identified as a "serious criminal case" and further investigation is under way.
Arson attacks by disgruntled people took place in the past as a bus in Chengdu, capital of southwestern Sichuan Province, was set ablaze by an unemployed man with gasoline in 2009. Twenty eight people were killed and more than 70 others injured in that incident.
Guo asked the medical officers to race against time to rescue the injured and ensure them best and timely treatment.
The Public Security Ministry has urged local police departments to take rapid and effective measures to carry out a safety overhaul and ensure overall social stability.
The ministry also demanded police to ensure the ongoing national college entrance exams, which started on Friday and will end tomorrow with more than nine million examinees.