Prison authorities in Nebraska—where executions were briefly banned three years ago—used the opioid fentanyl to carry out a death sentence for the first time in U.S. history. Carey Dean Moore, 60, who had been on death row for 38 years after being convicted of killing two Omaha taxi drivers in 1979, was executed with a controversial four-drug cocktail that included fentanyl, a powerful drug that’s received widespread attention amid the devastating opioid crisis that’s led to thousands of deaths in the U.S. each year. This was Nebraska’s first execution since 1997. It’s unclear why fentanyl in particular was used to carry out Moore’s death sentence, but Nebraska Corrections Director Scott Frakes has said in a court filing that execution drugs “are difficult, if nearly impossible, to obtain,” The New York Times reported. The move has sparked concern among advocates and prompted a federal lawsuit from the drug company Fresenius Kabi, which accused Nebraska officials of obtaining execution drugs “through improper or illegal means.” Nebraska authorities said the drugs were purchased lawfully.