“We had a strong antidiversion system in place, but no system is perfect,” Mr. Barrett said. Among other steps, the company said it had created a special committee to regularly evaluate pharmacies that order high volumes of narcotic drugs.

Another major distributor, AmerisourceBergen, recently disclosed that it faces a federal criminal inquiry into its oversight of painkiller sales. And in June, West Virginia officials filed a lawsuit against 14 drug distributors, including Cardinal and AmerisourceBergen, charging that they had fed illicit painkiller use in that state. The companies have denied wrongdoing.

D.E.A. officials have heralded the Cardinal action as the forerunner of a more aggressive approach to the painkiller problem. But critics say that for years, the agency did little to scrutinize distributors who were making tens of millions of dollars from the prescriptions generated by pain clinics in Florida, Ohio and other states. These facilities, often described as “pill mills,” employed doctors who wrote narcotics prescriptions after cursory examinations of patients.

“In the case of West Virginia, they have done nothing,” said a lawyer in Charleston, James M. Cagle, who is working on the state’s action against distributors.

The drug distribution system is a sprawling one that involves about 800 companies, which range in size from a few giants like Cardinal to hundreds of small firms. For wholesalers, the markup on medications can be small, sometimes a few pennies a pill. But with billions of pills sold annually, the profits can be big. Narcotic painkillers are now the most widely prescribed drugs in the United States, with sales last year of $8.5 billion.

This is not the first time the industry has faced scrutiny. In 2008, Cardinal paid $34 million to settle charges that it failed to alert the D.E.A. to suspicious orders for millions of pain pills that it was shipping to Internet pharmacies — operations that for years supplied the illicit market. The same year, another big distributor, McKesson, paid $13 million to settle similar charges. As part of the agreements, both companies denied wrongdoing.

Executives like Mr. Barrett of Cardinal say that it is often difficult for a distributor to tell whether a pharmacy or a doctor is serving legitimate pain patients or supplying illicit drug demand. And distributors have long complained that the D.E.A. has never issued specific guidelines for when they should stop shipping to a customer.