Don Sak district police chief Col Sutthi Nitiakkarapong said tests on both driver Samrueng Chuenchom, 36, and his assistant Amornthep Apairat, 22, found methamphetamine (ya bah) substances in their urine.
The tests were carried out by Don Sak Hospital after the double-decker bus operated by Lompraya Co skidded off Road 4142 in Don Sak district in Surat Thani on Monday.
The accident slightly injured Amornthep and American passenger Mariana Paredes, 34.
The driver said the bus was taking 35 tourists from Krabi to a Lomprayah pier in Don Sak, where they were to board a ferry to Koh Samui.
Col Sutthi said the driver and his assistant have been charged with having narcotic substances in their bodies, and their cases will be forwarded to state prosecutors in the province today (Jan 3).
They could face fines of up to B40,000 and an unspecified jail term if found guilty, according to Manop Sutthiphan, the chief of the land transport provincial office.
The transport official said Lomphayah had informed his office that both of them had been fired following the accident.
Surat Thani Governor Wichawut Jinto ordered the land transport office to examine possible legal against Lomphayah, the bus operator.
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