WASHINGTON – Seven U.S. senators sharply criticized the Drug Enforcement Administration on Wednesday for failing to answer questions about enforcement actions against pharmaceutical companies accused of violating laws designed to prevent painkillers from reaching the black market.

“We received an insufficient response that ignored those questions almost entirely and recited boilerplate information about the DEA’s mission,” said the letter to the acting DEA administrator, Chuck Rosenberg.

The DEA did not immediately respond to a request for comment.

The senators first sent the questions to the DEA in October after The Washington Post reported that the agency had slowed its enforcement efforts in the face of the opioid epidemic, which has cost nearly 180,000 lives since 2000. In 2015 alone, about 16,000 people died of prescription opioid overdoses, according to the Centers for Disease Control and Prevention.

The Post reported that beginning in 2013, DEA lawyers at headquarters started to delay and block enforcement efforts against large opioid distributors and others, requiring investigators in the field to meet a higher burden of proof before they could take action.

Five former supervisors from the DEA’s Diversion Control Division told The Post in on-the-record interviews that they had become increasingly frustrated by the sharp drop in enforcement actions. They said their civil cases were suddenly being subjected to a higher standard of proof, similar to that required in criminal cases.

The supervisors’ concerns were confirmed by reports filed by the DEA’s chief administrative law judge, who found that the number of cases approved at headquarters was “stunningly low for a national program.”

DEA officials have declined to publicly explain why the agency’s enforcement efforts have plummeted and why cases made by the agency’s field offices against pharmaceutical companies have languished, in some cases for years.

On Dec. 2, DEA officials held an off-the-record briefing for the senators. But the lawmakers said in the letter sent Wednesday that the session was not a “substitute for written responses to our questions.”

The senators said they wanted to know whether civil filings had fallen and if so, why. They also asked whether the Department of Justice played a role in policy changes that have affected the DEA’s enforcement efforts.

“Since we wrote to you, the Centers for Disease Control and Prevention (CDC) has released new data showing that the U.S. opioid epidemic is growing and that prescription and illicit opioids remain a driving force,” the senators wrote Wednesday. “Especially in light of this new data, Congress and the American people deserve an explanation of how the DEA is enforcing laws that could help address this public crisis.”

The letter was signed by Democratic Sens. Edward J. Markey of Massachusetts, Richard J. Durbin of Illinois, Joe Manchin III of West Virginia, Amy Klobuchar of Minnesota, Tammy Baldwin of Wisconsin and Richard Blumenthal of Connecticut and independent Bernie Sanders of Vermont.

“We do not yet know if the DEA is doing its job to prevent deadly prescription opioid painkillers from getting into the wrong hands as a result of decreased enforcement efforts against wholesale pharmaceutical distributors,” Markey said in a statement Wednesday.

Sens. Patrick J. Leahy, D-Vermont, and Ron Wyden, D-Oregon, sent a similar letter to Rosenberg and U.S. Attorney General Loretta E. Lynch after the Post stories. Wyden said Wednesday the response was insufficient.

“The DEA’s response to our inquiry was totally inadequate given the life-and-death nature of the opioid crisis,” he said.