CHICAGO (AP) — An Illinois sheriff’s office investigating a pediatrician who killed himself last year says tests have revealed several of his former patients lack immunity to diseases against which they or their parents believed they had been vaccinated.

Cook County Sheriff Tom Dart’s office said Monday that tests on former patients of Dr. Van Koinis of Evergreen Park found they lack immunity.

Authorities launched an investigation after finding a suicide note in which Koinis suggested he complied with some parents’ requests not to vaccinate their children. Dart’s office said Koinis had been licensed to practice in Illinois since 1991.