A new study that investigated the cause of a large outbreak of E. coli in 2009 has blamed raw chocolate chip cookie dough.

The researchers say it is the first time an outbreak of food poisoning caused by the dangerous Shiga toxin-producing E. coli has been traced to store-bought, ready-to-bake cookie dough or a similar product. T

he outbreak, between March and July 2009, sickened at least 80 people across 30 US states, 35 of whom had to be hospitalized.

In the new study, published in the latest issue of the journal Clinical Infectious Diseases, several of the people who were sickened bought the contaminated cookie dough with the sole intention of eating it raw. "They had no plans to actually bake cookies," the authors reported.

The study, led by Dr. Karen Neil, a medical epidemiologist with the Centers for Disease Control and Prevention, found that in this case it is likely that it was the flour to blame.

"Out of all the ingredients, raw flour is the only raw agricultural product that was in the cookie dough," Dr. Neil said. "It didn't undergo any specific processing to kill pathogens, so we feel that's the most likely suspect for what may have introduced contamination into the cookie dough. We couldn't prove it conclusively, but that's what we suspect."