WASHINGTON, D.C. - Attorney General Jeff Sessions announced charges Thursday against more than 400 medical professionals in what he called the largest health care fraud enforcement operation in U.S. history.

The indictments are part of National Health Care Fraud Takedown Week, an initiative by the U.S. Department of Health and Human Services.

Four-hundred-twelve physicians, nurses, pharmacists and other medical professionals were charged with fraud, accounting for more than $1.3 billion across more than 20 states.

One of those indicted is a former head of the Indiana State Chiropractic Association. Dr. Ronald Sheppard, chiropractor and owner of Castleton Integrative Health in Indianapolis, is accused of encouraging other chiropractors to refer patients to certain pharmacies, from which he then got monetary kickbacks.

The federal investigation also included charges for opioid scams, incolving the illegal prescription and distribution of narcotics.

Six Michigan doctors are accused of a scheme to prescribe unnecessary opioids.