(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.

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