(CNN) A Chicago-area pediatrician's suicide note has prompted an investigation into his vaccination practices and record keeping, according to the Cook County Sheriff's Office.

Dr. Van Koinis, found dead of suicide in September 2019, left behind a note that "raised questions about the record keeping of vaccinations at his medical practice," the sheriff's office said in a statement.

Based on the investigation, including "issues presented in the note," authorities have been unable to determine which of Koinis' patients were vaccinated and which were not, the statement said.

"Investigators also obtained information that suggests Dr. Koinis, a pediatrician, in some cases did not provide vaccinations to children at their parents' request," the sheriff's office said.

"Out of an abundance of caution, Dr. Koinis' former patients are encouraged to discuss this information with their current physicians and inquire about methods to test for prior vaccinations," the sheriff's office said.

For years, Koinis was a "very well-respected, well-liked doctor," Cook County Sheriff Thomas Dart told CNN affiliate WGN. But the note he left behind "was very clear that he had horrible regrets for how" he handled vaccinations over the course of 10 years, Dart said.

Koinis' practice was based in a building at 3830 West 95th Street in Evergreen Park, Illinois, the sheriff's office said.

His patients were primarily from the city's southwest side and the neighboring suburbs. He had been licensed to practice medicine in Illinois since 1991, the sheriff's office said, citing records from the state.

Testing could be 'elaborate and expensive'

All children from kindergarten through 12th grade are required to have received certain vaccines under Illinois state law.

There is the option of exemption on religious grounds, but the children's parents must present schools with a certificate detailing the grounds for exemption. That certificate must be signed by a health care provider who has confirmed they explained the benefits of immunizations and the risks to the child and the wider community.

According to Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center, doctors could determine through a series of blood tests whether the patients have antibodies for diseases they were supposed to have been vaccinated for, such as measles, mumps or chicken pox.

But it would require a lot of blood tests, Schaffner told CNN, and would be "very elaborate and expensive."

"It would be much easier to just revaccinate," he said. Vaccinations would be given over the course of several weeks, he said, depending on how many a patient needed.

There is precedent for being revaccinated, he said, though this case is certainly unusual.

"Problems with vaccine storage and handling come up with some frequency," Schaffner said, "and the response is to get revaccinated."

"It's unusual to have an entire record be unreliable," as in this case, he added, "so in effect those patients have to get revaccinated from scratch."