WATERLOO, Iowa (AP) — Three people who were attending a monster truck rally in Waterloo were taken to a hospital Saturday night with symptoms of carbon monoxide poisoning.

Waterloo Fire Rescue Battalion Chief Ben Petersen told the Waterloo-Cedar Falls Courier that truck exhaust was assumed to be the source of the carbon monoxide that sickened the spectators.

The three people became ill around 9 p.m. Saturday when the crowd was filing out of the National Cattle Congress complex at the end of the event.

The monster truck rally featured several top trucks including Bounty Hunter, Scarlet Bandit, Rat Attack and Hot Tamale. The event also featured a kids power wheel car race at intermission.