One member of the investigation team conducts investigation on spot after a tourist bus crashed and burned in Taoyuan, Taiwan, 19 July 2016. [Photo/IC]

Taiwan authorities ordered some tour buses off the road Wednesday after a coach fire killed 26 passengers, the AFP reports, as questions swirl over why emergency exits failed to open.

Four buses belonging to the Mei Kui Shih Transportation Company -- which operated the tour bus -- were ordered off the road, Taiwan's highways department said.

Another 16 buses of the same model, used by other tour companies, will undergo compulsory inspections in the next week, according to the department.

Taoyuan district prosecutors questioned the bus operator and travel agency Tuesday night and were continuing to investigate Wednesday.

They told AFP they were so far unable to speculate on the cause of the accident.

"(We) will conduct a second examination of the scene and vehicle to identify the cause," they said in a statement.

A post-mortem will be carried out on the body of the driver to determine the state of his health, the statement added.

The Taiwan tour bus carrying visitors from the Chinese mainland crashed into a highway railing en route to Taoyuan airport, the island's main airport, on Tuesday afternoon.

Local media reported that charred bodies were found piled at the escape routes on the bus, which caught fire and then crashed through a highway barrier.

Investigators believe mechanical or electrical failures near the driver's seat may have led to the disaster, according to reports.