An increasing number of drivers involved in fatal crashes are testing positive for drugs, especially opioids and marijuana, according to a new study by the Governors Highway Safety Association — though it is unclear whether drug use is actually the culprit in those crashes.

The report raises serious concerns at a time when the U.S. is facing an epidemic of opioid usage and as more and more states legalize marijuana for medical and recreational usage. But the GHSA also cautions that it is difficult to fully understand the extent to which “drugged driving” is becoming a problem.