A dentist in Colorado, who is currently suspended, reused needles and syringes in his now closed dentist practice, that potentially exposed thousands of his patient to hepatitis and HIV, said state health officials.

Letters were sent to over 8,000 patients of Stephen Stein from the Department of Public Health in Colorado. The letters urged the recipients to have tests for the diseases after the state had learned of the unsafe injection policies at two dentist clinics in the Denver-area owned from September of 1999 to June of 2011.

Investigators said that Stein reused syringes and needles in a number of patients’ intravenous hookups at his dental implant and oral surgery clinics, a direct violation of medical protocol. A prepared statement from the Health department said that no cases had been confirmed of anyone contracting the infections through the clinics owned by Stein.

The letter’s sent out to Stein’s both the current and former patients, urged the recipient to have testing done for hepatitis B and C along with HIV. Officials said that Stein’s records were not complete so more people are likely to have been exposed than the ones already identified.

Legal counsel for Stein said he was cooperating fully with the ongoing investigation. Stein’s dentist license was suspended for another matter that was unrelated. The reason for the current suspension must remain confidential until the probe for it is complete.

A spokesperson for the office of the U.S. Attorney said that Stein was a target of one criminal probe for possible fraud with prescriptions before the latest allegations emerged, although no charges have been filed to date.