KFSN-TV reported that health officials are investigating 20 reported E. coli cases – 16 from the same source – in an outbreak that apparently began after two graduation parties on May 19.

Food from the same Fresno market was served at both graduation parties.

A number of food sources are under scrutiny, but the parties were catered by the same company, said Tim Casagrande, the county’s director of environmental health.

Health workers are talking to people in attendance at two other parties held on the same day to determine if more people have become ill.