TAIPEI (AFP) - The Taiwan authorities ordered some tour buses off the road on Wednesday (July 20) after one caught fire and killed 26 on board, as questions swirl over why emergency doors and windows did not open.

Local media reported that bodies were found stacked at the escape routes at the back of the bus full of Chinese tourists, which caught fire then crashed through an expressway barrier on Tuesday on its way to Taipei's main airport in Taoyuan.

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

The China Times said preliminary investigations had also found emergency door handles may have been distorted by heat from the inferno inside the bus.

Pictures from the scene showed two men outside the bus trying to smash windows open with fire extinguishers.

One eyewitness said trapped passengers inside the bus had been pounding on the windows when it careered off the road.

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 have a compulsory inspection within the next week, the department said.

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

They told AFP they were unable as yet to speculate on the causes 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.

Chinese officials are due to land on Wednesday afternoon and arrangements are being made for family members to come to Taiwan, according to the island's Mainland Affairs Council.

Relatives will undergo DNA tests to help identify the bodies, prosecutors said.

The tour group of 24 visitors from China's north-eastern city of Dalian were heading to the airport for their flight home after an eight-day trip around the island when the bus caught fire, swerving sharply before ramming into the expressway barrier.

All died, including three children, along with the driver and tour guide who were both Taiwanese.