Federal officials are investigating an outbreak of Escherichia coli O157: H7 infections. Although the U.S. Centers for Disease Control and Prevention (CDC) says the cause is unknown, it also says recalled romaine lettuce has tested positive for the outbreak strain of the pathogen.

Earlier this month, California-based Tanimura & Antle recalled whole romaine heads from 20 states. The recall came after the Michigan Department of Agriculture and Rural Development found E. coli in the company’s romaine as part of its routine sampling program.

Both CDC and the U.S. Food and Drug Administration (FDA) are investigating the outbreak. FDA reported that a cause has not been determined: “The strain of E. coli found in the Michigan sample is a third distinct strain not genetically related to the strains causing two distinct multi-state outbreaks of Shiga-toxin producing E. coli O157: H7 that FDA and CDC announced on October 28. At this time, a specific food has not been linked to either of those outbreaks.”

While no deaths have been reported, illnesses span California, Illinois, Michigan, Ohio, Pennsylvania, and Virginia. For more information, visit the websites for FDA and CDC.