(CNN) -- Ohio inmate Dennis McGuire appeared to gasp and convulse for roughly 10 minutes before he finally died Thursday during his execution by lethal injection using a new combination of drugs, reporters who witnessed it said.
He was convicted in 1994 of the rape and murder of 22-year-old Joy Stewart, who was seven months pregnant. Her relatives were at Southern Ohio Correctional Facility in Lucasville to witness his death, according to tweets from television reporter Sheila Gray.
McGuire's "children and daughter-in-law were crying and visibly upset," Gray tweeted.
A Dayton Daily News reporter who also watched the execution said that before the drugs took effect, McGuire thanked Stewart's family for a letter he apparently got. "I'm sorry," McGuire reportedly said.
The convicted murderer was pronounced dead at 10:53 a.m. ET,
The execution generated controversy because, like many states, Ohio has been forced to find new drug protocols after European-based manufacturers banned U.S. prisons from using their drugs in executions -- among them, Danish-based Lundbeck, which manufactures pentobarbital.
The state used a combination of the drugs midazolam, a sedative, and the painkiller hydromorphone, the state corrections department told CNN.
In an opinion piece written for CNN earlier this week, a law professor noted that McGuire's attorneys argued he would "suffocate to death in agony and terror."
"The state disagrees. But the truth is that no one knows exactly how McGuire will die, how long it will take or what he will experience in the process," wrote Elisabeth A. Semel, clinic professor of law and director of the Death Penalty Clinic at U.C. Berkeley School of Law.
According to a pool report from journalists who witnessed the execution, the whole process took more than 15 minutes, during which McGuire made "several loud snorting or snoring sounds."
Ohio ran out of pentobarbital in September, according to JoEllen Smith, the spokeswoman for the Ohio Department of Rehabilitation and Correction.
In response to that shortage, the department amended its execution policy to allow for the use of midazolam and hydromorphone.
Stewart's body was discovered by hikers near a creek in southwestern Ohio in February of 1989. Her throat was cut and she had been sodomized.
The state was set to execute death row inmate Ron Phillips using the new drugs last year, but Gov. John Kasich granted the convicted killer a stay of execution pending a review of a possible organ donation to his family members.
There are currently 139 men and one woman on death row in Ohio, according to the corrections department website.