The group of local tourists had been returning from a trip to see seasonal cherry blossoms at a farm when their bus veered off a highway and flipped on its side on the outskirts of capital Taipei last month.
Occupants, many of them elderly, were tossed out as the roof ripped off.
It was the latest in a series of deadly incidents in Taiwan after a bus fire last July killed 25 Chinese holidaymakers.
It was travelling at 79 kph when it went off the carriageway.
"The road section is labelled with speed limits of 50 km, and 40 km at four different places," a statement from Shi-Lin District Prosecutors Office said.
Dashcam footage from the car travelling behind the bus was used in the investigation, they said.
Footage shown by local media straight after the crash from a vehicle following the bus shows it turning off the main highway and flipping over, leaving behind a mangled pile of metal.
It did not address whether the driver had been overworked, which his daughter has said was the case.
The driver had two outstanding traffic violations, including one for not wearing a seatbelt, but no drunk-driving record, the transport ministry said at the time.
There is a separate ongoing investigation into the travel agency and bus company, prosecutors said.
An investigation into last year's fatal bus inferno outside Taipei found the driver had intentionally set fire to it in a suicide bid before it veered into a crash barrier.
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