Genetic testing revealed the same E. coli bacteria was present in the people, the beef and the herd from which the steer was taken, state health officials said.
"Absolutely, it was the beef," Dr. John Lumpkin, head of the state Public Health Department, said Wednesday. "The most likely scenario is that the meat was not cooked completely through."
Lumpkin said E. coli from the steer's intestines cattle often carry the bacteria probably contaminated the meat when the animal was slaughtered and butchered.
The beef was served at a party held Labor Day weekend near Petersburg, about 25 miles northwest of Springfield. Of the more than 300 people who were sickened, 22 required hospitalization.
The E. coli strain can lead to intense cramps and bloody diarrhea. In severe cases, it can destroy red blood cells and interfere with kidney function.