Ten monks were killed after an 11-year-old crashed a truck into a pilgrimage group in Mukdahan province, with the death toll rising from earlier reports of nine.
Thirty-five monks had set out on a 260-kilometer walk to Ubon Ratchathani about 30 minutes earlier when the truck left the road and hit them.
Five monks died at the scene and five at a hospital; more than 10 others were hospitalized, with one still in critical condition.
Security footage showed the monks walking single file along the roadside, and witnesses told police the vehicle was swerving before it slid off the road.
The boy is in custody, the crash remains under investigation, and police said his parents could face negligence charges.