Between 2010 and August 2013, the doctor provided certification even though at least 20 percent of the patients were not confined to their homes, and Medicare was thus billed for treatments Davida knew were not medically necessary, prosecutors said.

In a plea agreement, Davida acknowledged that he feared that if he didn’t provide the certifications, the agencies would stop sending him referrals, prosecutors said.

Medicare paid the agencies more than $20 million based on orders signed by Davida. Because 20 percent of those patients were not confined to the home, the agency lost at least $4 million, prosecutors said.

The Medicare Fraud Strike Force, which consists of agents from the FBI and HHS, conducted the investigation with prosecutors from the U.S. attorney’s office and the Justice Department’s Fraud Section.