A sign with a DEA badge marks the entrance to the Drug Enforcement Administration Museum in Arlington. (Jonathan Ernst/Reuters)

DESTRUCTIVE AND persistent, the epidemic of opioid addiction is also deeply ironic: Unlike previous drug scourges in U.S. history, this one spread via perfectly legal channels. Millions of people were introduced to addictive pain-killing medications by doctors’ prescriptions, filled at pharmacies, ultimately supplied by pharmaceutical manufacturers. All of this went on in one of the most heavily regulated sectors of the U.S. economy — health care — which is supervised by a veritable army of officials working for dozens of agencies, state and federal.

Understandably, Americans want to know how the corporate, governmental and medical establishment could have presided over the growth of a public health crisis that caused 18,893 overdose deaths related to prescription pain relievers in 2014, plus 10,574 from a follow-on heroin epidemic, according to the Centers for Disease Control and Prevention. Some of the answers — or, at least, more good questions — emerged from a recent series of articles in The Post about the interaction between two little-known but crucial players in the pharmaceutical business: wholesalers that link factory to pharmacy, and the Drug Enforcement Administration, which is responsible for ensuring that legally produced narcotics aren’t diverted for improper or illegal use.

The key finding of the articles by Lenny Bernstein, Scott Higham and David S. Fallis is that DEA career personnel responded to the rise in opioid abuse after the turn of the 21st century by pursuing aggressive civil enforcement against wholesalers suspected of pumping drugs into corrupt “pill mills” around the country. These efforts were important because they enabled DEA officials to stop abusive practices through administrative orders, backed by financial penalties, without mounting full-blown criminal prosecutions. Yet the DEA officials told The Post they eventually ran into resistance from higher-level Justice Department officials who were being heavily lobbied by the wholesalers, which include some of the biggest corporations in the United States. The number of civil case filings against wholesalers plunged from 131 in fiscal 2011 to 40 in fiscal 2014 before rebounding to 64 in fiscal 2016.

Whether this decline is due to regulatory capture of the DEA by the industry, as the DEA’s in-house critics allege, better conduct by industry, as industry says, or a combination of the two is a matter Congress should investigate. Sens. Ron Wyden (D-Ore.) and Patrick J. Leahy (D-Vt.) sent Attorney General Loretta E. Lynch a letter asking her for a detailed response to the evidence in the Post stories — something neither Justice nor the DEA was willing to provide when our reporters asked. Congress too may have some explaining to do; this year it enacted, and President Obama signed, a law that makes it more difficult for the DEA to initiate civil enforcement actions against wholesalers, by raising the legal standard for doing so — all in the name of better “cooperation” between industry and government and patient “access” to necessary medicines.

Given the way addictive prescription drugs flooded into the market even during the days when wholesalers were complaining that the DEA was overly adversarial toward them, we’re skeptical that what’s called for is a friendlier industry-government relationship. To the contrary.