Captain Jackson Manatha said the bus was carrying passengers from Durban to King William's Town and it left the road at Kei Cuttings at around 5am on Thursday.
Four people were seriously injured and nine others lightly injured. The driver was unharmed.
All the injured were taken to the Butterworth hospital.
Some passengers who had escaped injury told police that they suspected that the driver had been fatigued and that speed could also have been a contributing factor.
A case of reckless and negligent driving was being investigated by Kei Bridge police, said Manatha.
Sapa
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