The state of Ohio put a man to death using a never-tried, lethal combination of drugs, a Department of Corrections official confirmed to The Nation.

Dennis McGuire, convicted for the 1989 rape and aggravated murder of Joy Stewart, died at 10:53 am today. He was 53.

Associated Press reporter Andrew Welsh-Huggins, who witnessed the execution, said it took more than fifteen minutes for McGuire to die. Another reporter at the scene, Alan Johnson of the Columbus Dispatch, observed:

At about 10:33 a.m., McGuire started struggling and gasping loudly for air, making snorting and choking sounds that lasted for at least 10 minutes, with his chest heaving and his fist clenched. Deep, rattling sounds eminated from his mouth. For the last several moments before he was pronounced dead, he was still.

Welsh-Huggins added that, “Previous executions with the former execution drugs took much less time, and typically did not include the types of snorts and gasps that McGuire uttered.”

A federal judge denied Monday a request to delay the execution, and both Governor John Kasich and the Ohio Parole Board rejected clemency pleas. Attorneys argued that the untried combination of drugs used to execute McGuire—the sedative midazolam and painkiller hydromorphone—could potentially violate his right to be free from cruel or unusual punishment.

David Waisel, an anesthesiologist at Harvard Medical School, testified in court last week that McGuire faced “a substantial risk of experiencing the terror of air hunger during the first five minutes of the execution.… Air hunger is a horrible feeling. It’s an inability to get your breath.”

The state of Ohio switched to the new lethal combination after a Danish manufacturer stopped allowing its drugs for executions.

Critics say experimenting with new methods pose additional risks on top of chances that execution technicians botch a job. Kevin Werner, executive director of Ohioans Against Executions, told The Nation, “Ohio is putting itself in a risky situation. If there weren’t problems with Dennis McGuire’s execution, there will be in the future. That’s the track record.”

In May 2007, execution technicians stuck Christopher Newton, a 265-pound man, ten times before finally getting a needle in place. It took more than two hours to complete his execution. In 2009, former Ohio Governor Ted Strickland had to halt the execution of Romell Broom due to technicians’ inability to find a suitable vein for lethal injection. He remains on death row to this day.

Dennis McGuire’s execution was the first of six scheduled in Ohio this year.