A Missouri lawmaker is spearheading an investigation of five drug manufacturers to prove their alleged role in America’s opioid epidemic.

Nearly 180,000 people have fatally overdosed on prescription opioids since 2000, and prescriptions for the drugs have increased almost three-fold throughout the same time period.

Data released by the Centers for Disease Control and Prevention last December found that the prescription opioids responsible for the most deaths are strong painkillers such as oxycodone, morphine, methadone, and hydrocodone. The strength of Oxycodone, which is prescribed under the name Oxycontin, is reportedly comparable to heroin.

Missouri Senator Claire McCaskill, the ranking Democrat of the Homeland Security and Governmental Affairs Committee, seems to believe opioid makers reward doctors for excessively prescribing their products and withhold information about the risk of addiction. On Tuesday, March 28, McCaskill recently sent letters to Purdue Pharma, Janssen Pharmaceuticals, Insys Therapeutics, Mylan, and Depomed demanding information on sales and marketing materials, according to the Washington Post.

She wrote:

“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. To achieve this goal, manufacturers 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.”

McCaskill’s letters also request information regarding studies the companies might have performed on addiction and the sums they have donated to advocacy groups. In addition to some of the most widely-prescribed opioids in the US, Mylan is the maker of the EpiPen, an injection medication for potentially fatal allergic reactions. The company was accused of price-gouging last year after raising the price of the medication by approximately $500. A cheaper, generic EpiPen was released in response to the scandal, which noted that Mylan rakes in over $11 billion in revenue a year.

McCaskill’s request comes just days after a new study determined that most American teenagers who abuse prescription opioids first received the drugs from doctors. Researchers from the University of Michigan examined trends in prescription opioids from 1976 to 2015 and soon discovered that while teens initially take the drugs for medical reasons, they eventually start taking them for “non-medical” reasons.

Another recent study conducted at the University of Minnesota revealed that over 25,000 pregnant women ages 18-44 who currently abuse opioids received the drugs from doctors as well. Sean McCabe, professor and leader of the University of Michigan study, suggested prescribing lower doses of opioids and supplementing them with mild pain medications such as ibuprofen to curb the epidemic.

A few weeks ago, Senator McCaskill asked the Justice Department’s inspector general to find out why the Drug Enforcement Administration (DEA) has apparently ceased efforts to launch cases against wholesale opioid distributors that may be violating certain legal guidelines. In her request, McCaskill noted that DEA attorneys have raised the bar for what can be considered grounds for a case, despite the undeniable increase in overdose rates for the drugs involved.

Numerous US counties have sued drug manufacturers due to the outrageous costs of managing the opioid crisis. These lawsuits demand reimbursement for these expenses, claiming the manufacturers are almost entirely responsible for the drugs falling into the wrong hands.