E. coli bacteria found on frozen leftover meat perfectly matches the bacteria found in stool samples taken from several people who became ill, said Dr. Mark Lundberg, Butte County health officer.
He said it's still unclear how the meat became contaminated.
Hundreds of people attended the barbecue on Sept. 6, which raised money for the volunteer fire department in Forest Ranch. Afterward, a number of people became sick with severe stomach cramps and diarrhea. Four people got so ill they had to be hospitalized.
Health officials learned that the sickness was caused by a dangerous strain of E. coli bacteria called E. coli 0157:H7.
Interviews with people who attended the event led officials to conclude the tri-tip was contaminated and made people sick. However, they couldn't be sure until the link was proven by test results from the state lab, Lundberg said in a phone interview on Monday.
It's still not known how the cooked meat became contaminated, he said, and it may never be known.
Food preparers at the event had the right equipment and, according to interviews, seemed to do everything right, he said, but obviously something went wrong.
When large amounts of food are prepared there is the potential for contamination, he said. It's possible the cooked meat came into contact with juices from the raw meat. Or possibly, he said, someone who helped prepare the food was sick and didn't wash his or her hands properly.
People who are planning to serve food at public events are welcome to call the Public Health Department to obtain guidelines for safe food preparation, he said. The number to call is 538-7281.