At least 61 people are now ill with E. coli O157:H7 linked to a picnic held by Neff’s Lawn Care in Germantown, Ohio, according to the Dayton & Montgomery County Health Department.

Of those sickened, 11 have been hospitalized and 11 have tested positive for the same strain of E. coli, said Bill Wharton, spokesman for the health department.

The cause of the outbreak remains unknown as health officials continue their investigation. Between 200 and 300 people attended the event, and the health department suspects more individuals may report illnesses.

Those who attended the event and fell ill are encouraged to contact the health department at 937-225-4460.