An 11-year-old boy was arrested after allegedly crashing a pickup truck into a group of Buddhist monks and pilgrims in northeastern Thailand on Thursday, killing at least eight monks, according to officials and police.

The monks were taking part in a pilgrimage on foot from a temple in Mukdahan province toward Ubon Ratchathani province. Authorities said five monks died at the crash site and others later died from their injuries at a hospital. In addition to the fatalities, multiple people were injured and taken to medical facilities, including some in serious condition.

Police said the cause of the crash is still under investigation. Reports from witnesses and initial accounts indicated the monks saw the vehicle lose balance before it left the road and hit the group. The boy is believed to be in custody while investigators and child-protection officials review what happened.