The incident occurred around midnight Thursday as the bus was traveling from Khon Kaen province to Chiang Rai province in the country's rural region, CNN reported.
A police official, speaking to CNN, quoted investigators as saying they believe the vehicle's driver may have been unfamiliar with the region and perhaps was speeding.
The report said the bus went off the Huay Tong bridge after leaving long skid marks.
Other details were not immediately available.
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