The Justice Department charged more than 600 people, including 165 doctors and other medical professionals, with making $2 billion in false billings in what Attorney General Jeff Sessions said Thursday was the nation’s largest ever health care fraud takedown.

The cases are connected to the nation’s ongoing drug overdose crisis, which killed nearly 64,000 people in 2016, Sessions said. About two-thirds of the overdose deaths were caused by opioids, led by illicit fentanyl.

“It is the deadliest drug epidemic in the history of this country,” Sessions said at a news conference. “We have never seen anything like it. Some of our most trusted medical professionals look at their patients, vulnerable people suffering from addiction, and they see dollar signs.”

Of those arrested, 162 defendants, including 76 doctors, were charged for their roles in prescribing and distributing opioids and other dangerous narcotics.

“Many of these fraudsters have stolen tax dollars, and many have helped flood our streets with drugs,” Sessions said. “One doctor allegedly defrauded Medicare of more than $112 million by distributing 2.2 million unnecessary dosages of drugs like oxycodone and fentanyl.” Last year, the Justice Department charged more than 400 people across the country with participating in health care fraud scams totaling about $1.3 billion in false billings, including for the prescription and distribution of opioids.

The defendants are accused of having participated in schemes to submit claims to Medicare, Medicaid, TRICARE, the military’s health care program, and private insurance companies for treatments that were medically unnecessary and often never provided, according to court documents.

In many cases, patient recruiters, beneficiaries and others were allegedly paid cash kickbacks in return for supplying beneficiary information to providers so the providers could then submit fraudulent bills to Medicare, court documents say.

In one indictment in the Central District of California, an attorney allegedly paid kickbacks and offered prostitutes and expensive meals to two podiatrists in exchange for prescriptions written on preprinted prescription pads, regardless of the medical need for the prescriptions, according to court documents. Once the prescriptions were filled, people involved in the alleged scheme submitted about $250 million in fraudulent claims to federal, state and private insurers for drugs, the documents say.

Drug Enforcement Administration Assistant Administrator John Martin said a nine-month investigation in Nashville, led to the indictments of five people who were part of an oxycodone prescription drug trafficking ring. They allegedly obtained 4,800 oxycodone pills for distribution, he said. In Baton Rouge, the DEA took down another drug trafficking organization accused of running a pill press operation.

“They were churning out 40,000 counterfeit oxycodone pills per week,” Martin said.

Health and Human Services Secretary Alex M. Azar said investigators have found people setting up fake pharmacies, “harvesting seniors’ Medicare information and coming up with new ways every day to steal from program beneficiaries and taxpayers.” One of the defendants allegedly used his position at a recovery center to prescribe controlled substances without a license while the center worked in tandem with other treatment centers to “bilk those trying to enter recovery,” he said.

“They attack programs helping some of the most vulnerable, those struggling with addiction, because they know those are the ones that have the most good will, ” Azar said. “The perpetrators really are despicable and greedy people.”