The U.S. Department of Justice joined lawsuits accusing opioid-maker Insys Therapeutics of paying kickbacks to doctors to prescribe a powerful opioid medication, in an effort to generate more profit.

According to a filing made public on Monday in federal court, the Justice Department became part of the litigation against Insys. It follows a wave of related criminal cases against medical practitioners, former executives and sales representatives employed by Chandler-based Insys, including its billionaire founder John Kapoor.

Federal and state prosecutors have charged the company with bribing doctors to prescribe the drug beyond its approved use. Insys has said it’s been trying to negotiate a settlement of a federal probe into its marketing of fentanyl-based Subsys, a synthetic opioid spray, 100 times stronger than heroin, designed only for use in breakthrough pain.