A Senate committee is investigating whether practices at the top five makers of opioids in the United States fueled an epidemic of painkiller abuse that has led to the fatal overdoses of tens of thousands of Americans.

Sen. Claire McCaskill, D-Mo., on Tuesday sent letters to the companies seeking information about sales and marketing materials, internal studies on addictions, details on their compliance with legal settlements and donations to advocacy groups.

The companies are Purdue Pharma, Johnson & Johnson‘s Janssen division, Insys, Mylan and Depomed. McCaskill is the ranking Democrat on the Senate’s Homeland Security and Governmental Affairs Committee.

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The companies did not immediately respond to CNBC requests for comment.

“The investigation will explore whether pharmaceutical manufacturers — at the head of the opioids pipeline — have contributed to opioid over-utilization and over-prescription as overdose deaths in the last 15 years have approached nearly 200,000,” said a press release announcing the probe.

The statement notes that in 2015 alone, more than 30,000 people died from overdoses of either prescription opioids or of heroin.

And since 1999, the release says, sales of prescription opioids have increased four-fold.

“This epidemic is the direct result of a calculated sales and marketing strategy major opioid manufacturers have allegedly pursued over the past 20 years to expand their market share and increase dependency on powerful — and often deadly — painkillers,” McCaskill wrote in her letters to the companies.

“To achieve this goal, manufactures have reportedly sought, among other techniques, to downplay the risk of addiction to their products and encourage physicians to prescribe opioids for all cases of pain and in high doses,” she wrote.

McCaskill said she often hears stories about “drug overdose deaths, the vast majority of them related to prescription opioids or heroin,” which are “single-handedly destroying families and communities across Missouri and the country.”

“And I refuse to just stand by and watch. We have an obligation to everyone devastated by this epidemic to find answers,” McCaskill said. “All of this didn’t happen overnight. It happened one prescription and marketing program at at time.”

Her letters note that in 2007, Purdue Pharma paid $635 million in fines to settle criminal and civil charges related to the company’s misrepresentation of the addictive qualities of its OxyContin painkiller medication, and that three executives of the firm pleaded guilty to criminal misbranding.

They also note that the city of Chicago sued Purdue Pharma, Janssen, and several other opioid makers in 2015, claiming the companies deceptively marketed their painkillers.

The Chicago lawsuit alleges that Purdue and Janssen have sponsored continuing medical education activities for doctors “that were delivered thousands of times promoting chronic opioid and supporting and disseminating … deceptive and biased messages.”

The sessions “inflate the benefits of opioids, and frequently omit or downplay their risks or adverse effects,” the complaint claims.

Purdue and Janssen have denied the claims.