Frank Younker, a veteran DEA supervisor, has said that his Cincinnati field office could not get its cases against big opioid distributors through the agency’s headquarters. (Matt McClain/The Washington Post)

Seven more senators demanded information Friday about a steep decline in enforcement actions by the Drug Enforcement Administration against large wholesale companies that distribute opioid painkillers.

Six Democrats and one independent expressed “serious concern” that “over the last few years, the [DEA] has scaled back its enforcement efforts” against distributors who violate laws designed to prevent painkillers from falling into the hands of illicit drug users.

In a four-page letter to Attorney General Loretta E. Lynch and DEA acting Administrator Chuck Rosenberg, the senators cited the findings of a Washington Post investigation published last weekend.

“The allegations in the Washington Post article are especially troubling given the opioid-abuse epidemic that is claiming nearly 30,000 lives in the United States annually,” they wrote.

The letter was signed by Democratic Sens. Edward J. Markey (Mass.), Richard J. Durbin (Ill.), Joe Manchin III (W.Va.), Amy Klobuchar (Minn.), Tammy Baldwin (Wis.), Richard Blumenthal (Conn.) and independent Bernie Sanders (Vt.). On Wednesday, Sens. Patrick J. Leahy (D-Vt.) and Ron Wyden (D-Ore.) sent a similar request to Lynch.

As the worst of a nationwide opioid epidemic raged in Appalachia, DEA investigators went after companies distributing millions of highly addictive pills. Then, their cases ground to a halt. (Lee Powell/The Washington Post)

A Justice Department spokesman said the department would review the letter. The DEA did not immediately respond to an email requesting comment.

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

About 165,000 people died of overdoses caused by prescription narcotics between 2000 and 2014, and tens of thousands more succumbed to overdoses of heroin and fentanyl, according to the Centers for Disease Control and Prevention.

Five former supervisors from the DEA’s Diversion Control Division told The Post that they were frustrated by the sharp drop in enforcement actions. The supervisors’ concerns were echoed in reports filed by the DEA’s chief administrative law judge.

Investigators said they unsuccessfully confronted Clifford Lee Reeves II, the attorney in charge of the DEA unit that approves administrative cases against people and companies suspected of diverting painkillers to the black market.

On Friday, the senators asked whether actions against painkiller distributors and others had indeed dropped from 131 in 2011 to 40 in 2014 — the Justice Department figures cited by The Post — and if so, why. They requested an explanation for the higher standard imposed on investigators in the field and sought information about whether companies had turned to then-Deputy Attorney General James M. Cole for relief from DEA efforts.