A tour bus catches fire on highway to the Taiwan Taoyuan International Airport in Taiwan, July 19, 2016. [Photo by China Times/Asianewsphoto]

The preliminary investigation into the Taiwan bus fire which claimed 26 lives has shown that the blocking of emergency exit led to the tragedy, the local Central News Agency (CNA) reported.

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

All 26 on board were killed at the scene, including 24 mainland tourists on an eight-day tour organized by a travel agency in Northeast China's Liaoning province, and the driver and the tour guide, both from Taiwan.

Eyewitnesses said the bus had been giving off smoke and swerving from lane to lane prior to crashing and bursting into flames.

Video from the scene showed both of the bus' doors were pressed against the highway's guard rail, making them impossible to open.

Thirteen badly charred bodies were found piled in front of the bolted emergency exit, indicating the panicked passengers had rushed to open the door, but failed.

The rest of the bodies were found on the aisle, with only few on the seats. A fire-extinguisher equipped on the bus lay unused.

"The fire moved very fast," Lu Jui-yao, an official with the local highway police bureau, said.

One passenger even managed to put one of his feet out of the rear door, but the crack was too narrow for an escape.

The sudden engulf of the flames must have resulted from the high-speed driving, strong wind, and flammable objects on the coach such as the filling foams of the chair.

However, there was no official word on the cause of the fire. Some travel agency insiders doubted that the driver forgot to loosen the hydraulic brake system so the overheating finally led to a brake failure.

Others speculated the circuit fire erupted from the water dispenser and Karaoke equipment near the driver's seat.

The case is under investigation.