Buses in China often have surveillance cameras.
All those onboard are presumed dead.
But the new footage shows the driver was being hit by a passenger and then struck her back.
"The investigation showed that a 48-year-old female passenger, surnamed Liu, planned to get off at a certain stop, but the bus had to make a detour and would not arrive there due to road maintenance", reported China's state-run Xinhua news agency.
CCTV from on-board the bus shows how a passenger appeared to lash out at the driver, who in turn retaliated, seconds before the bus crashed and plunged into the Yangtze River.
China is still reeling from a tragic incident in which a bus swerved off a bridge, plummeting 60 meters into the icy waters of the Yangtze river, killing all 15 people on board but now shocking footage has revealed the cause. The driver turned his head back several times arguing.
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The two exchanged gunfire, he said, and Bowers reentered the building before a police tactical squad arrived. He did not have a criminal record and was not known to police, law enforcement officials said on Saturday.
A crane salvages the wreckage of a bus after it plunged off a bridge into the Yangtze River in China's southwestern Chongqing early on November 1, 2018.
After the driver refused to stop, Liu hit the driver's head with her cellphone. The driver's right hand came off the steering wheel during the altercation, which led to the steering wheel turning sharply to the left, the report said.
During the ensuing fight, the driver lost control and the bus went into the river.
Rumours prompted calls to ban female drivers, as the auto that the bus hit was driven by a woman.
Thirteen bodies were found after the Sunday night crash, and two people remained missing, Chongqing Police told China Daily Friday morning.