​(USA TODAY) -- Federal authorities charged more than 400 people in what Attorney General Jeff Sessions called the largest health care fraud takedown in U.S. history.

The attorney general said that suspects accounted for more than $1.3 billion in fraudulent transactions, and at least 120 were involved in opioid-related crimes, making it also the largest-ever opioid-related fraud takedown.

Acting FBI Director Andrew McCabe said the 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 "once-trusted" doctors and pharmacists played major roles in the fraud.

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