A quarter century ago, Seattle-based litigator Bill Marler spent a lot of time representing consumers who fell ill after eating hamburger contaminated with E. coli.

Today, Marler is in the early stages of litigation against one of the most famous fast-food burger chains in the country: Wendy’s Co. His firm Marler Clark LLP has already filed six lawsuits against Wendy’s in the Midwest, and more are coming.

It’s unlikely, however, that contaminated hamburger is to blame for nearly 100 illnesses. Instead, in an outbreak whose source has not yet been confirmed, many people have reported becoming sick after eating sandwiches with romaine lettuce at Wendy’s restaurants in Indiana, Michigan, Ohio and Pennsylvania, according to the Centers for Disease Control and Prevention (CDC).

As of a Sept. 1 posting, CDC reported 97 illnesses and 43 hospitalizations in an E. coli outbreak with an unknown food source. Eleven people fell ill in Indiana, 1 in Kentucky, 58 in Michigan, 24 in Ohio, 1 in New York and 2 in Pennsylvania, according to the disease prevention agency.

“Investigators are working to confirm whether romaine lettuce is the source of this outbreak, and whether romaine lettuce used in Wendy’s sandwiches was served or sold at other businesses,” CDC stated. “Wendy’s is fully cooperating with the investigation.”

As investigators continue digging to identify the source of the outbreak, Wendy’s faces a growing number of lawsuits filed on behalf of people who have fallen ill. Marler said in an interview that he’s representing 39 people, and he confirmed he plans to file dozens more lawsuits on behalf of his clients.

One of the lawsuits was filed on behalf of Ebone Colbert-Taylor in Oakland County Circuit Court in Michigan. On July 27, she ate “a Dave’s Single hamburger with all the toppings,” according to an amended complaint. Four days later, Colbert-Taylor “began experiencing the symptoms of an E. coli O157:H7 infection, including diarrhea, nausea and vomiting,” the lawsuit alleged.

She subsequently sought medical treatment and while hospitalized developed a condition known as hemolytic uremic syndrome (HUS), the complaint said. According to the Mayo Clinic, HUS can occur when the small blood vessels in someone’s kidneys become damaged and inflamed and is often caused by certain strains of E. coli.

The lawsuit filed against Wendy’s and its franchisee alleges strict liability, breach of warranty, negligence, negligence per se and violation of the Michigan Consumer Protection Act. It seeks the award of damages and attorney’s fees and costs.

Ultimately, as more details around the source of the outbreak are uncovered, others in the supply chain are susceptible to being sued as well.

“There most likely will be additional defendants added,” Marler said. He said later in the interview, “There’s clearly a link to Wendy’s here. The real question is what was the vector and if there is a clear vector, is it enough to bring the supply chain into the litigation?”

Marler said it’s likely the outbreak is tied to lettuce, though early signs indicate other food may have been cross-contaminated. Two of Marler’s clients reported eating chicken nuggets and one of his clients fell ill from a family member who ate at Wendy’s, he said.

As of Friday afternoon, Wendy’s had not responded to a request for comment.

Escherichia coli (E. coli) generally reside in the intestines of animals and people, but some types of E. coli can cause adverse symptoms and be transmitted through contaminated food or water or through contact with animals or people, according to CDC. Diarrhea—frequently bloody—severe stomach cramps and vomiting are among the symptoms of Shiga toxin-producing E. coli, the agency reports on its webpage about the outbreak tied to Wendy’s.

These days, however, E. coli is rarely associated with hamburger. “Although E. coli linked to hamburger used to be where I did most of my work, that has not been the case for the last decade and a half,” Marler shared.