Senator Clair McCaskill (D-MO), the top ranking Democrat on the Senate Homeland Security and Governmental Affairs Committee, has expanded the probe into America's opioid addiction crisis, the Wall Street Journal reports.

McCaskill requested that four more drug companies and three drug distributors turn over documents about the steps the companies have taken to keep prescription painkillers from getting into the black market.

On Wednesday the Senator sent letters to drug makers Mallinckrodt (MNK) , Endo International (ENDP) , Teva Pharmaceutical Industries (TEVA) , and Allergan (AGN) . She also reached out to distributors McKesson Corp. (MCK) , AmerisourceBergen Corp. (ABC) , and Cardinal Health (CAH) .

"We've seen numerous reports that potentially hundreds of millions of opioid pills wound up on the black market, fueling a nationwide epidemic," Sen. McCaskill said in a statement, the Journal added. "We need a better understanding of how committed these companies are to preventing this type of drug diversion or whether they are turning a blind eye."

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