OAKLAND, Iowa (AP) — Local and federal investigators are trying to determine why a fire erupted on a school bus in western Iowa, killing a student and the bus driver. The fire occurred Tuesday morning after the bus picked up a student at a farm southeast of Oakland. Sixteen-year-old Megan Klindt and 74-year-old Donnie Hendricks died in the fire.

Fire investigators and representatives of the National Transportation Safety Board examined the charred remains of the bus Wednesday. John Reynolds, a chief deputy with the Pottawattamie County Sheriff’s Office, says a timeline for the fire investigation hasn’t been determined.

NTSB spokesman Keith Holloway says the agency may issue its preliminary findings within 30 days, but its full investigation could take at least a year.