SANTA CLARITA (CBS) — A mystery substance found on a school bus at a park in Santa Clarita prompted a hazardous-materials response on Wednesday.

Firefighters responded to a call at approximately 12:23 p.m. at Canyon Country Park in the 17000 block of Soledad Canyon Road, according to Los Angeles County Fire Department.

About 60 students from Canyon Springs Elementary School had been on a field trip to the San Fernando Mission, where one of them found the balloon, according to county fire Capt. Mark Savage.

The balloon later burst as some students were playing with it, according to Savage. The call for help came when nobody could identify the contents of the balloon and may have suspected it was heroin, which is often packed in balloons.

Firefighters determined the powder was harmless flour for baking.

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