WASHINGTON (AP) — Companies selling some of the most lucrative prescription painkillers funneled millions of dollars to advocacy groups that in turn promoted the medications' use, according to a report released Monday by a U.S. senator.

The investigation by Missouri's Sen. Claire McCaskill sheds light on the opioid industry's ability to shape public opinion and raises questions about its role in an overdose epidemic that has claimed hundreds of thousands of American lives. Representatives of some of the drugmakers named in the report said they did not set conditions on how the money was to be spent or force the groups to advocate for their painkillers.

The report from McCaskill, ranking Democrat on the Senate's homeland security committee, examines advocacy funding by the makers of the top five opioid painkillers by worldwide sales in 2015. Financial information the companies provided to Senate staff shows they spent more than $10 million between 2012 and 2017 to support 14 advocacy groups and affiliated doctors.

The report did not include some of the largest and most politically active manufacturers of the drugs.

The findings follow a similar investigation launched in 2012 by a bipartisan pair of senators. That effort eventually was shelved and no findings were ever released.

While the new report provides only a snapshot of company activities, experts said it gives insight into how industry-funded groups fueled demand for drugs such as OxyContin and Vicodin, addictive medications that generated billions in sales despite research showing they are largely ineffective for chronic pain.

"It looks pretty damning when these groups were pushing the message about how wonderful opioids are and they were being heavily funded, in the millions of dollars, by the manufacturers of those drugs," said Lewis Nelson, a Rutgers University doctor and opioid expert.

The findings could bolster hundreds of lawsuits that are aimed at holding opioid drugmakers responsible for helping fuel an epidemic blamed for the deaths of more than 340,000 Americans since 2000.

McCaskill's staff asked drugmakers to turn over records of payments they made to groups and affiliated physicians, part of a broader investigation by the senator into the opioid crisis. The request was sent last year to five companies: Purdue Pharma; Insys Therapeutics; Janssen Pharmaceuticals, owned by Johnson & Johnson; Mylan; and Depomed.

Fourteen nonprofit groups, mostly representing pain patients and specialists, received nearly $9 million from the drugmakers, according to investigators. Doctors affiliated with those groups received another $1.6 million.

Most of the groups included in the probe took industry-friendly positions. That included issuing medical guidelines promoting opioids for chronic pain, lobbying to defeat or include exceptions to state limits on opioid prescribing, and criticizing landmark prescribing guidelines from the U.S. Centers for Disease Control and Prevention.

"Doctors and the public have no way of knowing the true source of this information and that's why we have to take steps to provide transparency," said McCaskill in an interview with The Associated Press. The senator plans to introduce legislation requiring increased disclosure about the financial relationships between drugmakers and certain advocacy groups.