(CNN) -- Ohio inmate Dennis McGuire appeared to gasp and convulse for roughly 10 minutes before he died Thursday by lethal injection using a new combination of drugs, reporters who witnessed it said.
McGuire 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.
She said McGuire, before the drugs took effect, thanked Stewart's family for a letter he apparently received.
"To my children, I'm sorry. I love you. I'm going to heaven and I'll see you there when you come," McGuire reportedly said, according to CNN affiliate WDTN.
Columbus Dispatch reporter Alan Johnson said that the whole execution process took 24 minutes, and that McGuire appeared to be gasping for air for 10 to 13 minutes.
"He gasped deeply. It was kind of a rattling, guttural sound. There was kind of a snorting through his nose. A couple of times, he definitely appeared to be choking," WDTN quoted Johnson as saying.
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.
According to Ohio's corrections department, the state used a combination of the drugs midazolam, a sedative; and the painkiller hydromorphone.
Both the length of time it took for McGuire to die and his gasping are not typical for an execution, said Howard Nearman, an anesthesiologist at University Hospitals Case Medical Center in Cleveland.
"Why it took 24 minutes, I really can't tell you," he said. "It just makes you wonder -- what was given? What was the timing, and what were the doses?"
In an opinion piece written for CNN 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.
Speaking on behalf of McGuire's legal team, attorney Allen Bohnert called on the governor to impose a moratorium on future executions because of what took place Thursday.
"At this point, it is entirely premature to consider this execution protocol to be anything other than a failed, agonizing experiment," he said in a statement.
"The people of the State of Ohio should be appalled at what was done here today in all of our names. Ohio, like its citizens, must follow the law. The state has failed."
CNN's Sonny Hostin said that McGuire's execution will likely spark debate over whether how inmates react to the use of the drugs constitutes cruel and unusual punishment prohibited by the U.S. Constitution.
"Whenever there's a change in the lethal injection process clearly it's subject to legal proceedings and perhaps we will see those," Hostin said.
Ohio ran out of pentobarbital, which is a narcotic and sedative barbiturate, in September, according to JoEllen Smith, 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.
There are currently 138 men and one woman on death row in Ohio.
The state was set to execute death row inmate Ron Phillips using the new drug combination 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.
CNN's Joe Sutton, Ross Levitt and Deborah Feyerick contributed to this report.