Anthony Boyd, 54, was executed Thursday in Alabama by nitrogen gas for his role in the 1993 killing of Gregory Huguley over a $200 drug debt.

Boyd maintained his innocence in his final words, criticizing the criminal justice system and expressing love for those fighting for change.

The execution appeared to take longer than previous nitrogen gas executions, with Boyd showing movements and heavy breathing for over 15 minutes before being pronounced dead.

Legal challenges to halt the execution or allow him to die by firing squad were denied by both a federal judge and the U.S. Supreme Court, despite dissent from Justice Sonia Sotomayor, NBC News has reported.

Alabama has defended the method, asserting that observed movements during executions are involuntary reactions to oxygen deprivation, while critics, including Boyd’s spiritual adviser, called the process painful and mishandled.