(Reuters) - U.S. Democratic Senator Claire McCaskill on Thursday expanded an investigation into the causes of the opioid crisis plaguing the country, seeking information from four more drugmakers and three drug distributors.

FILE PHOTO: Ranking Member Sen. Claire McCaskill (D-MO) speaks prior to Homeland Security Secretary John Kelly testifying before a Senate Homeland Security and Governmental Affairs hearing on border security on Capitol Hill in Washington, D.C., U.S., April 5, 2017. REUTERS/Aaron P. Bernstein

The top-ranking Democrat on the Senate Homeland Security and Governmental Affairs Committee is focusing on the distribution of opioids and the efforts companies made to report and investigate the diversion of drugs for illicit use.

The investigation comes at a time when lawmakers and regulators take steps to combat the epidemic of opioid addiction, with 91 Americans dying everyday as a result of overdose, according to the U.S. Centers for Disease Control and Prevention.

McCaskill requested for documents and information from opioid manufacturers Mallinckrodt Plc, Endo International Plc, Teva Pharmaceutical and Allergan Plc.

In March, McCaskill asked Johnson & Johnson, Mylan NV, Purdue Pharma, Insys Therapeutics Inc and Depomed Inc for internal estimates of the risk of abuse, addiction and overdose of opioids.

The senator also sent a request to McKesson Corp, AmerisourceBergen Corp and Cardinal Health Inc, focusing on their distribution of opioid products.

McCaskill also requested for information on compensation provided that is in any way derived from revenue or profitability targets for sales of opioid products.

“We appreciate Senator McCaskill’s inquiry and look forward to thoughtfully answering her questions,” a McKesson spokeswoman said in an email, adding that there was no link between the sale of controlled substances and incentive compensation for the company’s executives.

AmerisourceBergen said it would respond to the letter from the Senator, adding it provides daily reports to the regulators about the quantity, type and receiving pharmacy of every single order of controlled substances it distributes.

Endo said it received the letter and would provide the Senator with the requested information.

Allergan said it is working with the Senator’s office to provide the requested information. The company also said its two branded opioid products – Norco and Kadian – account for 0.1 percent or less of all opioids prescribed in the United States.

Israel-based Teva in an email said it is committed to the “appropriate use of opioids and responsible pain management”.

Mallinckrodt confirmed receipt of Senator McCaskill’s request for information, adding it has previously met with and provided information to her staff and would continue working with her office. The company noted it manufactures “only generic and non-promoted branded opioids”.

Cardinal Health declined to comment.