Sen. Claire McCaskill (D-Mo.) speaks on Capitol Hill. In a letter Monday, she called for the Justice Department’s inspector general to look into a slowdown in DEA enforcement amid the opioid crisis. (Alex Brandon/AP)

A U.S. senator on Monday called for an investigation into why the Drug Enforcement Administration slowed enforcement efforts against pharmaceutical companies accused of violating laws designed to prevent pain pills from reaching the black market.

Sen. Claire McCaskill (Mo.), the ranking Democrat on the Senate Homeland Security and Governmental Affairs Committee, in a letter asked the Justice Department’s inspector general to investigate. She said she had “serious concerns” about reports of an enforcement slowdown as the opioid epidemic escalated nationwide.

“This a matter of life and death and I want to know whether or not we could have done more,” McCaskill said in a statement.

A spokesman for Inspector General Michael E. Horowitz declined to comment. A DEA spokesman said the agency would review the senator’s request.

McCaskill cited recent reports by The Washington Post that the agency had delayed and blocked enforcement efforts against several large opioid distributors and required field investigators to meet a higher burden of proof before they could take action.

Five former supervisors from the DEA’s Diversion Control Division, which regulates the pharmaceutical industry, told The Post in on-the-record interviews that they had become frustrated by the slowdown as overdose deaths soared, particularly in the nation’s rural communities.

Their concerns were documented by the DEA’s chief administrative law judge, who said in his quarterly reports to agency supervisors that the number of enforcement actions being approved at headquarters was “stunningly low for a national program.”

The Post also reported that enforcement efforts had been questioned by high-ranking Justice Department officials and that key architects of the DEA’s campaign against opioid distributors had been hired away by the pharmaceutical industry.

McCaskill noted that 183,000 people have died from overdoses of prescription narcotics between 1999 and 2015, with more than 15,000 in 2015 alone. She said her home state ranked first among Midwestern states in the number of prescription painkillers being sold.

The senator said that the DEA had reached financial settlements in several cases­ against the nation’s largest distributors of narcotic painkillers. But she questioned whether those penalties were sufficient to curb the continuing epidemic and hold the companies accountable.

The DEA’s efforts have been “too little, too late,” she said in the letter requesting the investigation.

In October, nine other U.S. senators demanded that the DEA explain why it has slowed enforcement efforts against the pharmaceutical industry. In letters to the agency, those senators said their questions have been “ignored” by the agency or that the answers they received “were totally inadequate.”