Federal authorities announced charges Thursday against 412 physicians, nurses, pharmacists and other medical professionals, in what Attorney General Jeff Sessions called the largest health care fraud enforcement operation in U.S. history.

Sessions said suspects accounted for more than $1.3 billion in fraudulent transactions across more than 20 states, and at least 120 people were charged for their alleged roles in over-prescribing and distributing opioids, making it also the largest-ever opioid-related fraud takedown.

Of the 412 charged in the year-long operation, 56 were physicians.

"Too many trusted medical professionals...have chosen to violate their oaths and put greed ahead of their patients,'' Sessions said. "Amazingly, some have made their practices into multi-million dollar criminal enterprises.''

A doctor at a Houston clinic is accused of writing 12,000 prescriptions for opioids, enough for more than 2 million illegal doses.

The enforcement effort comes as the country continues to battle a fatal epidemic of prescription drug abuse, much of it involving opioids. Abuse of the expensive painkillers often lead addicts to cheaper and often-lethal alternatives: heroin and fentanyl.

Last year, an estimated 59,000 people in the U.S. died from drug overdoses, many of them linked to opioid abuse, according to the Drug Enforcement Administration. Casualties are on pace this year to exceed 60,000, Sessions said.

Acting FBI Director Andrew McCabe said the overall enforcement operation involved 29 bureau field offices around the country and more than 300 agents.

"The nation is in the midst of a crisis," McCabe said. "Opioid abuse destroys lives."

McCabe said singled out the doctors, pharmacists and nurses as having violated the personal trust of their patients and clients who depended on them for well-being.

"These people,'' McCabe said, "inflicted a special kind of damage.''

He said investigators found opioid addicts "packed in standing-room-only waiting rooms" at doctors' offices waiting for their prescription painkillers.

"Some doctors were writing more prescriptions than entire hospitals," McCabe said.

In one case, a group of six Michigan doctors allegedly operated a scheme to provide patients with unnecessary opioid prescriptions and later billed Medicare for $164 million in false claims. Some of the those prescribed pain-killers, authorities said, were re-sold on the street to addicts.