A top Senate Democrat is launching a formal investigation into one of the big culprits behind the nation’s worst drug overdose crisis in history: pharmaceutical companies.

On Tuesday, Sen. Claire McCaskill (D-MO), the top Democrat on the Senate Homeland Security and Governmental Affairs Committee, announced that she is requesting marketing, sales, and addiction study material from the companies behind America’s top five opioid products. The investigation, she said, will draw out the role that opioid manufacturers played in causing the epidemic and letting it continue.

The opioid crisis is extremely urgent. In 2015 alone, more than 52,000 deaths were linked to drug overdoses, with nearly two-thirds of those deaths linked to opioids like OxyContin, Percocet, heroin, and fentanyl. And between 1999 and 2015, more than 560,000 people in the US died from drug overdoses — a death toll larger than the entire population of Atlanta.

It’s unclear just how far McCaskill’s investigation will go or whether Republicans on the committee are on board with her investigation. As USA Today noted, McCaskill will need Republican support on the committee to be able to subpoena opioid makers’ documents should the companies not comply with her requests.

But there’s a good reason for the investigation: Drug manufacturers played a major role in the epidemic. By marketing their opioid painkillers as safe and effective, they convinced doctors to prescribe painkillers in droves to patients. That allowed the drugs to proliferate, leading not just to widespread painkiller misuse but also to the misuse of more dangerous opioids like heroin and illegally manufactured fentanyl. With this, the risk of overdose increased — spawning the opioid crisis we have today.

Opioid makers played a big role in getting people hooked on drugs

Perhaps no single data point makes the connection between drugmakers and the opioid epidemic clearer than the following chart from an analysis published in the Annual Review of Public Health, which shows how increases in opioid sales went hand in hand with increases in opioid overdose deaths and drug treatment admissions:

Much of this was the result of misleading marketing by major drug companies.

In fact, Purdue Pharma, a producer of OxyContin, in 2007 was forced to pay hundreds of millions of dollars in fines due to its false claims about opioids. The Associated Press reported:

Purdue Pharma, its president, top lawyer and former chief medical officer will pay $634.5 million in fines for claiming the drug was less addictive and less subject to abuse than other pain medications, U.S. Attorney John Brownlee said in a news release… Purdue learned from focus groups with physicians in 1995 that doctors were worried about the abuse potential of OxyContin. The company then gave false information to its sales representatives that the drug had less potential for addiction and abuse than other painkillers, the U.S. attorney said.

In wide-ranging investigations, the Los Angeles Times has uncovered more evidence of how Purdue’s misleading advertisement played out at the ground level. It found that Purdue exaggerated the effectiveness and safety of OxyContin, while covering up any criticisms and complaints about the drug. As a particularly egregious example of why Purdue and its marketers did this, one sales memo uncovered by the Times was literally titled “$$$$$$$$$$$$$ It's Bonus Time in the Neighborhood!”

Other opioid makers have faced similar allegations. Insys, a drugmaker, allegedly pushed its fentanyl spray for uses far beyond late-stage cancer pain treatment, according to McCaskill’s office. A sales representative claimed that the company’s informal motto was, “Start them high and hope they don't die.”

Opioid companies have defended their practices in the past, arguing that they have taken steps to try to make their drugs less prone to abuse and altered guidance to physicians to that effect.

Still, drugmakers have lobbied against new restrictions on opioids, and their marketing played a huge role in convincing doctors that opioids were an effective, safe solution to pain. So doctors prescribed the drugs in record numbers, prompting a rapid rise in overdose deaths.

The opioid epidemic, explained

The 52,000 drug overdose deaths in 2015 were higher than the annual death tolls of any previous drug crisis, from the crack cocaine epidemic of the 1980s and ’90s to past heroin epidemics. To put the death toll in perspective, the 52,000 deaths are higher than the more than 38,000 who died in car crashes in 2015, the more than 36,000 who died from gun violence in 2015, and the more than 43,000 who died due to HIV/AIDS during that epidemic’s peak in 1995.

What makes the current drug crisis unique is that it began with legal drugs, marketed and sold by major pharmaceutical companies.

Back in the 1990s, doctors were persuaded to treat pain as a serious medical issue. There's a good reason for that: About one in three Americans suffer from chronic pain, according to a 2011 report from the Institute of Medicine.

Pharmaceutical companies took advantage of this concern. Through a big marketing campaign, they got doctors to prescribe products like OxyContin and Percocet in droves — even though the evidence for opioids treating long-term, chronic pain is very weak (despite their effectiveness for acute, short-term pain), while the evidence that opioids cause harm in the long term is very strong.

So painkillers proliferated, landing in the hands of not just patients but also teens rummaging through their parents’ medicine cabinets, other family members and friends of patients, and the black market.

As a result, opioid overdose deaths trended up — sometimes involving opioids alone, other times involving drugs like alcohol and benzodiazepines (typically prescribed to relieve anxiety). By 2015, opioid overdose deaths totaled more than 33,000 — close to two-thirds of all drug overdose deaths.

Seeing the rise in opioid misuse and deaths, officials have cracked down on prescriptions painkillers. Law enforcement, for instance, threatened doctors with incarceration and the loss of their medical licenses if they prescribed the drugs unscrupulously.

Ideally, doctors should still be able to get painkillers to patients who truly need them — after, for example, evaluating whether the patient has a history of drug addiction. But doctors who weren’t conducting even such basic checks are now being told to give more thought to their prescriptions.

Yet many people who lost access to painkillers were still addicted. So some who could no longer access prescribed painkillers — or perhaps could no longer afford them — turned to cheaper, more potent opioids: heroin and fentanyl, a synthetic opioid that's often manufactured illegally for nonmedical uses.

Not all painkiller users went this way, and not all opioid users started with painkillers. But statistics suggest many did: A 2014 study in JAMA Psychiatry found many painkiller users were moving on to heroin, and a 2015 analysis by the Centers for Disease Control and Prevention found that people who are addicted to prescription painkillers are 40 times more likely to be addicted to heroin.

So other types of deadly opioid overdoses, excluding painkillers, also rose.

That doesn't mean cracking down on painkillers was a mistake. It appeared to slow the rising number of painkiller deaths, and it may have prevented doctors from prescribing the drugs to new generations of people with drug use disorders.

But the likely solution is to get opioid users into treatment. According to 2014 federal data, at least 89 percent of people who met the definition for a drug use disorder didn't get treatment. Patients with drug use disorders also often complain of weeks- or months-long waiting periods for care.

So federal and state officials have pushed for more treatment funding, including medication-assisted treatment like methadone and Suboxone.

Some states, such as Louisiana and Indiana, have taken a “tough on crime” approach that focuses on incarcerating drug traffickers. But the incarceration approach has been around for decades — and it hasn’t stopped massive drug epidemics like the current opioid crisis.