Convicted killer Dennis McGuire's attorney Allen Bohnert called Thursday's execution in Ohio "a failed, agonizing experiment" after a never before used lethal drug combination made him suffocate for more than 20 minutes before dying, the Associated Press reported.

A couple of minutes before McGuire was injected with the two drugs, sedative midazolam and the painkiller hydromorphone, Gary Mohr, the Ohio prison director, said the drugs would lead to a "a humane, dignified execution" and would abide by the state's ban on cruel and unusual punishment, according to the AP.

McGuire was strapped to a gurney inside the prison's execution chamber as he thanked his victim's family for the kind words in a letter he received from them and said: "I'm going to heaven. I'll see you there when you come," the AP reported.

McGuire was convicted for the 1989 death of Joy Stewart and her unborn child after he raped and stabbed her to death, according to the AP.

In a reverse set of motions, McGuire lay very still for the first couple of minutes the drugs were injected intravenously. He then made a snorting noise and began struggling to breath for more than 10 minutes, according to the AP.

Usually, the person will move after being injected and will become motionless after the drugs take effect, the AP reported.

After the execution finished at 10:53 a.m. on Thursday, almost 25 minutes after McGuire had been injected, Bohnert said in a statement that "the people of the state of Ohio should be appalled at what was done here today in their names," the AP reported.

Since reinstating capital punishment in 1999, the state had used a different drug combination which did not take as long to kill the person and caused none of the sounds made by McGuire, but supply ran out after the manufacturer said it could no longer be used to carry out executions, according to the AP.

Several tries at postponing McGuire's execution had been presented to the courts in the past weeks by his attorney, including a plea to allow McGuire to donate his organs and an argument stating the untried method could cause "air hunger," a phenomenon which causes suffocation and feelings of "agony and terror," according to the AP.

Ohio's new injection procedure will be used five more times in 2014, with the next execution set for Feb. 19, the AP reported. JoEllen Smith, a spokeswoman for the Ohio prison, said a normal review of the execution will be conducted but made no further comment.