Through a spokesman, Eric Phillips, Ms. Rice declined to say whether criminal charges might be considered against the anesthesiologist, Dr. Harvey S. Finkelstein.

"Given the urgent need to identify exactly what happened and to fully understand the scope of the possible threat, the district attorney made the decision to launch an immediate investigation," Mr. Phillips said.

Neither Dr. Finkelstein nor his spokesman replied to messages yesterday requesting comment.

On Nov. 10, the State Health Department notified 628 of Dr. Finkelstein's patients that they might have been exposed to hepatitis C, hepatitis B, or H.I.V. from 2000 to 2005, as a result of Dr. Finkelstein's improper reuse of syringes in his clinic, which specializes in pain management. The department first confirmed the problem in January 2005 but took 34 months to notify patients.

The state health commissioner, Richard F. Daines, has recommended that anyone who received an injection from Dr. Finkelstein in his private practice, which began in the 1980s, should be tested for the two types of hepatitis and H.I.V. The recommendation came after hundreds of current and former patients contacted health authorities to ask if they should be tested. So far, hundreds of patients have had the blood tests; one had a positive result for hepatitis B, though health officials said further tests were needed to determine whether it was connected to Dr. Finkelstein's work.

In January 2005, State Health Department epidemiologists traced the transmission of a hepatitis C infection to his private clinic in Plainview. Dr. Finkelstein's case was reviewed by the Office of Professional Medical Conduct, but by the time the review took place, Dr. Finkelstein had changed his methods to comply with standard practice.

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