Sen. Claire McCaskill (D-Mo.) expanded her congressional investigation into opioid manufacturers and distributors, sending document requests to an additional four drugmakers.

McCaskill sent letters July 26 to the CEOs of Mallinckrodt, Endo, Teva and Allergan — as well as three distributors: McKesson, AmerisourceBergen and Cardinal Health.

The national overdose epidemic is a “direct result” of prescription opioid sales and marketing over the past two decades, she said after sending similar requests to the top-five opioid producers in March.