Romaine lettuce grown in Yuma, Ariz., may be the source of a widespread E. coli outbreak that has sickened students in three states, including University of Michigan students.

Federal investigators are looking at a farm in Yuma as a possible source for the outbreak, according to the distributor who sold the lettuce.

In Michigan, 10 cases of E. coli illness were confirmed, nine of which were in Washtenaw County. An additional three remain under investigation, the Washtenaw County health department said. More than 20 people reported symptoms consistent with E. coli, but some tested negative or couldn't be tested, a news release said. All of the confirmed illnesses occurred between April 9 and April 16.

Freshway Foods in Sidney, Ohio, is shown Friday. Federal investigators are looking at a farm in Yuma, Ariz., as a possible source of a widespread E. coli outbreak in romaine lettuce, according to the distributor. Freshway Foods of Sidney, Ohio, said Thursday it recalled lettuce sold in 23 states and the District of Columbia because of a possible link to an E. coli outbreak that has sickened at least 19 people.

Luke Gronneberg | The Sidney Daily News

Freshway Foods of Sidney, Ohio, said Thursday it recalled lettuce sold in 23 states and the District of Columbia because of a possible link to the E. coli outbreak that has sickened at least 19 people — three with life-threatening illness. The federal Centers for Disease Control and Prevention said it was looking at 10 other cases probably linked to the outbreak, which was caused by a rare strain of the disease.

College students at the University of Michigan, Ohio State in Columbus and Daemen College in Amherst, N.Y., are among those who were affected by the outbreak, according to health departments in those states. Several middle and high school students at four schools in Wappingers Falls, N.Y. and nearby Hopewell Junction, N.Y., also may have been sickened by the tainted romaine, according to a letter sent to parents from the local superintendent and posted on one school's website.

Health officials said most of the college-age victims were sickened in April and have already recovered. The May 6 letter from the New York superintendent, James Parla, said there were two confirmed cases, three probable cases and one suspected case of E. coli in the middle and high schools but no new cases had been reported since April 25. Parla said lettuce in the schools tested positive for E. coli and the cases were likely connected to those in Michigan and Ohio.

Freshway Foods Vice President Devon Beer said the company worked with the Food and Drug Administration to trace the contaminated lettuce to a Yuma grower, whom he would not identify.

The recall only applies to romaine lettuce with "best if used by" date before or on May 12, when Freshway Foods stopped buying its romaine from Yuma, Beer said.

Officials in Arizona also confirmed the investigation. Laura Oxley, a spokeswoman for Arizona's agriculture and health departments, said federal officials contacted them and told them they suspected the source of the E. coli outbreak was lettuce grown in the state. She said there were no additional shipments to stop because the winter lettuce season has mostly ended for the year.

The Yuma area is the source of much of the nation's winter lettuce crop, but farmers switch to other crops at the end of winter.

Freshway Foods said Thursday it was recalling romaine lettuce sold under the Freshway and Imperial Sysco brands. No contamination was found at the company's processing plant, according to the FDA. New York state's Public Health Laboratory discovered the contamination in a bag of Freshway Foods shredded romaine lettuce on Wednesday after local authorities had been investigating an outbreak for several weeks.

The most common strain of E. coli found in U.S. patients is E. coli O157. The CDC said the strain linked to the lettuce, E. coli 0145, is more difficult to identify and may go unreported.

E. coli infection can cause mild diarrhea or more severe complications, including kidney damage. The three patients with life-threatening symptoms were diagnosed with hemolytic uremic syndrome, which can cause bleeding in the brain or kidneys.

It was not immediately clear why students in schools and on college campuses made up the majority of victims in the outbreak. Freshway Foods said the lettuce was sold to wholesalers, food service outlets, in-store salad bars and delis. The recall also affects "grab and go" salads sold at Kroger, Giant Eagle, Ingles Markets and Marsh grocery stores. Bagged lettuce at the grocery store is not involved in the recall so far.

Most of the recalled lettuce was sold in states east of the Mississippi River. It was sold in Alabama, Connecticut, the District of Columbia, Florida, Georgia, Illinois, Indiana, Kansas, Kentucky, Maryland, Massachusetts, Michigan, Missouri, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee, Virginia, West Virginia and Wisconsin.