Officials on Tuesday said a double-decker bus carrying municipal workers on a field trip plunged off a steep road into a ravine in western Thailand, killing at least 30 people.

Tak provincial governor Suriya Prasatbunditya said another 22 people were injured by the crash that occurred when the driver tried to pass other cars on a steep, winding mountain road.

The bus slid off the mountain road and then flipped several times before crashing into a tree. The driver told authorities the brakes stopped working.

The bus was one of four carrying local workers and villagers on Monday night from Tak to northeastern Thailand.

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