The Centers for Disease Control and Prevention said Friday it is investigating an E. coli outbreak in four states that has sickened at least 37 people and put 10 in the hospital.

The health protection agency said the source of the outbreak has not been determined but said many of the sick people had reported eating sandwiches with romaine lettuce at Wendy's restaurants in Michigan, Ohio and Pennsylvania before getting sick.

So far, 19 people reported falling ill in Ohio, 15 in Michigan, two in Pennsylvania and one in Indiana, but the CDC said the true number is likely to be higher. There have been no reported deaths linked to the outbreak.

Wendy's said in a statement that is it "fully cooperating with public health authorities on their ongoing investigation" and was removing romaine lettuce from sandwiches in the region as a precautionary measure. The CDC said it was still working to confirm if the lettuce was the cause. Wendy's added that the romaine lettuce used in its salads is different from the lettuce in its sandwiches.

The CDC said there was no evidence that romaine lettuce from other restaurants or grocery stores is linked to the outbreak and was not advising people to stop eating at Wendy's.

Illnesses were reported from July 26 through Aug. 8, and the ages of sick people range from 6 to 91. Among the 10 hospitalized, three developed a type of kidney failure, the CDC said.

There are nearly 1,100 Wendy's restaurants in the four states, according to Reuters.