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.