The heads of some of the US’s largest corporations struggled to fight off accusations in Congress on Tuesday that they fueled the opioid epidemic by dumping millions of prescription painkillers on parts of the country worst hit by the crisis.

The CEOs of the three biggest drug distributors outright denied contributing to a disaster claiming tens of thousands of lives a year. But their position was undermined when the head of a smaller wholesaler broke ranks and acknowledged responsibility and failures. Another executive said he was “deeply sorry” for some of his company’s actions but still denied it had contributed to the escalating death toll.

The distributors faced withering criticism from Democrats and Republicans for allegedly ignoring legal requirements to closely monitor opioid deliveries to pharmacies while pouring vast amounts of painkillers into small towns in West Virginia, the state worst hit by the epidemic.

The distributors delivered 780m opioid pills to a state of just 1.8 million people over the five years to 2012. “How did the tiny town of Kermit with a population of just 400 receive 9m pills in two years?” asked Representative Diana DeGette.

The companies are not household names but they are among the richest in the country. They include McKesson, the US’s fifth largest corporation with nearly $200bn in revenue last year. Two other firms, AmerisourceBergen and Cardinal Health, are among the top 15 with revenues over $100bn.

Under sharp questioning by members of Congress exasperated by repeated evasions, the heads of the three companies said “no” when asked whether their firms contributed to the opioid epidemic – even though all three have paid fines to settle federal accusations they failed to obey laws requiring them to report and cut off opioid supplies to pharmacies dispensing excessively large amounts of narcotics. But Joseph Mastandrea, chair of a smaller firm, Miami-Luken, agreed his company was responsible.

The companies face a raft of lawsuits by states and cities that some lawyers say could result in financial settlements on a scale to match payouts by cigarette makers. Officials in some of the worst hit cities are also pressing the US justice department to pursue criminal prosecutions against top executives of the opioid manufacturers and distributors they describe as drug dealers in Armani suits.

The congressional hearing focused in part on why drug distributors filled huge orders for narcotic painkillers from small town pharmacies in West Virginia in the face of the growing death toll from the opioid epidemic and warnings from the Drug Enforcement Administration that they were legally obliged to report suspicious orders and halt deliveries.

Questioning focussed on supplies to a clutch of drugstores in south-western West Virginia where several doctors and pharmacists have since been imprisoned for running pill mills supplying opioids to people who sometimes travelled hundreds of miles to obtain drugs.

The CEO of McKesson, John Hammergren, was asked about his company’s delivery of 5.6m opioid pills to the Sav-Rite drugstore in Kermit in 2006-7. McKesson’s own monitoring system requires that deliveries larger than 8,000 opioid pills a month should be scrutinised. The company was delivering the equivalent of 9,650 pills a day to the Sav-Rite.

Hammergren admitted that deliveries “should have been terminated sooner” but put the failure to do so down to error.

But George Barrett, the CEO of Cardinal Health until last year and now its chairman, took a more conciliatory line in expressing “personal regret” at his company’s failure to cut off deliveries to two other pharmacies in the area.

“With the benefit of hindsight, I wish we had moved faster and asked a different set of questions. I am deeply sorry we did not,” he said.

Representative David McKinley, a Republican representing a district in West Virginia, accused the CEOs of feeding the epidemic by riding roughshod over the law. “None of you was complying with state law,” he said. “And yet you say we weren’t responsible. I think you were very much responsible.”

McKinley asked why, if doctors and pharmacists have gone to prison, drug distributor executives should not be jailed too. “I just want you to feel shame about your roles, respectively, in all of this,” he said.

Hammergren told Congress his company had struggled to understand the requirements. “In the past we’ve had challenges understanding the regulations the regulator was asking us to follow,” he said.

Although the firm continues to say it has done nothing wrong, McKesson last year paid a record $150m fine for violating the law requiring it to report suspicious orders, its second penalty for the same offense. The company said it paid the penalty “rather than engage in time consuming, contentious and expensive litigation”. Cardinal Health paid $44m to settle a similar case.