WASHINGTON — The Centers for Disease Control and Prevention said Friday that E. coli-contaminated ground beef was the suspected culprit for an outbreak that infected at least 109 people in six states.

Investigators are a step closer to tracking down the source of contamination that has perplexed them since the outbreaks began in early March. The CDC said 17 people have been hospitalized, though no deaths have been reported.

Health officials said they were still working to determine the source of tainted meat from stores and restaurants where people ate. No common source has been publicly identified.

Reported illnesses come from states that share at least one border with another in the group: Indiana, Ohio, Kentucky, Virginia, Tennessee and Georgia.

The CDC is not recommending people stop eating ground beef or that retailers stop selling it. But health officials have reminded people to handle raw ground beef safely and to cook it thoroughly.

Symptoms of E. coli infection often include severe stomach cramps, diarrhea and vomiting, typically occurring three days after consuming the bacteria.

This particular strain, E. coli O103, has been a relatively infrequent source of outbreaks. In 2010, an outbreak in Minnesota sickened 29 — one contamination among nearly 20 in the past two decades.

Federal health investigators interviewed dozens of affected people from the recent ground beef infections to determine its source. The widening dragnet indicates that health officials struggled at first to determine the source.

“Given the size and the number of states that are involved, what you’re seeing is very unusual,” Bill Marler, a food safety lawyer from Seattle, previously told The Washington Post.

Marler said the CDC estimates that for every person reported sick, there are five to 10 ill people who have not been accounted for.

Major foodborne outbreaks in the United States have more than tripled in the last 20 years, and the germs most frequently implicated are familiar to most Americans: salmonella, E. coli, and listeria. From 2010 to 2014, the most recent five-year period, these multistate outbreaks were bigger and deadlier than in years past, causing more than half of all deaths related to contaminated food outbreaks, according to a 2015 CDC report.

Tainted ground beef has been among the culprits. Authorities recalled a total of about 12 million pounds of raw beef products last year after more than 400 people in 30 states fell ill from salmonella infection. More than 100 people were hospitalized.

Other foods, ranging from vegetables and fresh fruits to chicken, have also helped spread infections nationwide. Some had never before been linked to outbreaks, such as the caramel apples, tainted with listeria, that led to an outbreak in which seven people died and 34 were hospitalized in late 2014.

Condiments, garnish, toppings and spices can all contain traces of E. coli. But it’s unlikely the patients in this outbreak were keeping track of all the additives in their recent diets, Marler told The Post.