A Northbrook chiropractor was sentenced Tuesday to 20 months in federal prison for billing an insurance carrier for services that were either unnecessary or weren’t provided at several north suburban chiropractic clinics that he co-owned.

Steven Paul, 46, and his co-defendant, Bradley Mattson, jointly owned six clinics in Hawthorn, Woodfield, Stratford, Algonquin, Niles and Cumberland, according to the U.S. Attorney’s Office for the Northern District of Illinois. Paul and Mattson admitted in plea agreements that they required patients to receive an initial x-ray and a prearranged schedule of clinic visits over a six-month period.

From 1999 to 2008, Paul directed billings to Blue Cross Blue Shield totaling $3.65 million for tests or physical therapy services that were medically unnecessary or weren’t provided, prosecutors said. As a result, his clinics collected $1.33 million in fraudulent reimbursements from the company.

Over the course of the investigation, an undercover FBI agent visited the pair’s Hawthorn clinic complaining of back strain, prosecutors said. Mattson diagnosed the agent with a pinched nerve and ordered a treatment plan that began with daily visits for two weeks. Mattson’s diagnosis was reportedly given despite the opinions of the clinic’s doctor and a physical therapist, who both said the agent was actually suffering from a pulled muscle.

Paul, who had previously pleaded guilty to one count of health care fraud, was sentenced Tuesday to 20 months in prison during a hearing in federal court in Chicago, prosecutors said. U.S. District Judge Ronald A. Guzman cited Paul’s “extraordinary cooperation” as the basis for imposing what he said was the shortest prison sentence he would consider for the fraud scheme.

Mattson, of Lake Forest, also pleaded guilty to health care fraud, prosecutors said. He was sentenced to over six years in prison in 2012.