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WASHINGTON — The Department of Justice on Thursday announced charges against more than 400 people in connection with a major crackdown on medical fraud aimed at combating the nation’s opioid epidemic.

The investigation focused on doctors who authorities say were prescribing unnecessary opioids and medical facilities that preyed on addicts to shell out cash for unnecessary treatments that only worsened their condition.

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Related: Lots of Americans Prescribed Opioids, Insurance Survey Shows

“Some doctors wrote out more prescriptions for controlled substances in one month than entire hospitals were writing,” Acting FBI Director Andrew McCabe told reporters. “To opioid addicts, these prescriptions escalate their dependence on drugs. They are a death sentence, plain and simple.”

The scams resulted in more than $1 billion in false billing uncovered in what Attorney General Jeff Sessions called “the largest health care fraud takedown operation in American history.” Much of that came out of the government's pocket by illegally billing Medicare and Medicaid.

Among those charged were a fake Florida rehab facility that is alleged to have used gift cards and strip club visits to recruit addicts, and a Houston doctor who sold thousands of opioid prescriptions for cash.

“We want to send a message to those that are violating the law that we are coming after you,” Sessions said.

The attorney general said the crackdown is part of President Donald Trump’s goal to curb the nation’s drug epidemic. The Centers for Disease Control and Prevention said earlier this month that prescriptions for painkillers have tripled since 1999.

“The United States is by far the highest prescribing opioid nation in the world, no nation is close to it,” Sessions said. “And I think that is a cause of addiction.”