Synthetic opioids are killing a skyrocketing number of people in the US, and the cause for the surging death rates is likely the growing trade in illegally manufactured fentanyl, according to new research by the Centers for Disease Control and Prevention. With more people dying of fentanyl overdoses now than ever before, according to the Drug Enforcement Agency (DEA), figuring out where that fentanyl is coming from will be critical to keeping people safe.

Fentanyl, which made headlines when Prince overdosed on it in April, is a lab-created synthetic opioid that’s 50 to 100 times more powerful than morphine. Doctors prescribe it as an injection, patch, lollipop, or nasal spray for patients with severe pain that can’t be treated with morphine. But outside the clinic, black market fentanyl can be mixed with or sold as heroin, or added to counterfeit prescription drugs without buyers knowing. It’s possible Prince overdosed by taking counterfeit hydrocodone pills laced with fentanyl, reported Stephen Montemayor with the Star Tribune.

Today’s study, published in the CDC’s Morbidity and Mortality Weekly Report, investigated what’s causing this surge in overdose deaths caused by synthetic opioids. Across 27 different states, the number of people killed climbed from 3,105 in 2013 to 5,544 in 2014. The most occurred in Massachusetts, Maine, New Hampshire, Ohio, Florida, Kentucky, Maryland, and North Carolina. In those states, non-hispanic white men between the ages of 25 and 44 were the most at risk.

Not every state tracks fentanyl deaths separately from deaths caused by other synthetic opioids, but in the six that did, the number of people who died of a fentanyl overdose climbed to 1,400 in 2014 from less than 400 in 2013 — nearly the same amount as the total increase in synthetic-opioid related deaths. This led the CDC to believe that fentanyl is driving the spike in synthetic opioid-related deaths.

To figure out where that fentanyl is coming from, the CDC analyzed prescription rates and drug seizures between 2013 and 2014. They found that prescriptions have remained steady even as death rates climb. But the number of drugs seized by law enforcement that tested positive for illegally manufactured fentanyl surged by 426 percent during that time period.

This means that the current epidemic, which the CDC calls "unprecedented in scope," isn’t being driven by illegally sold prescription drugs, but by a massive influx of illegally manufactured fentanyl masquerading as heroin and hiding in counterfeit prescription drugs. The DEA has identified China as the main source of the fentanyl and fentanyl-like compounds that find their way into the US.

The CDC calls for an urgent response to this spike, including more opioid surveillance, more testing for fentanyl and fentanyl-like compounds, and more access to the opioid overdose antidote naloxone.