BANGKOK — An 11-year-old boy crashed a pickup truck into a group of monks on a pilgrimage walk in northeastern Thailand on Thursday, killing nine, officials said.

A total of 35 monks from Mukdahan province, about 600 kilometers (372 miles) northeast of the capital Bangkok, were on the pilgrimage. Five monks were killed at the site, while four others died at a hospital. Thirteen were hospitalized with three in critical condition, according to the provincial administration.

The group started the 260-kilometer (161-mile) walk to Ubon Ratchathani province about 30 minutes before the crash.

Security camera footage shared by a local rescue group, Ruam Jai Mukdahan Rescue Association, shows the monks walking in a single line on the side of a road before the truck crashes into them.

The boy is in custody and will be questioned when state child protection officers arrive, police said.

The cause of the accident is under investigation, but police said the monks described seeing the vehicle swerving before it slid off the road and crashed into the group.