Donald Anthony Grant, 46, was pronounced dead at 10:16 a.m. CT at the Oklahoma State Penitentiary, a state corrections spokesman announced.
The execution began at 10:03 a.m., and Grant was declared unconscious at 10:08 a.m. before his death at 10:16 a.m., Corrections Department Director Scott Crow said.
There were 18 witnesses, including news reporters, prosecutors, a police chief and loved ones of Grant and his victims.
Grant's disjointed final words lasted two minutes before a prison staff member in the execution chamber stopped him and cut off the microphone.
"I got this. This ain't nothing," Grant said. "I'm solid, son. No meds, no nothing. I'm solid."
Grant kept speaking after the microphone was turned off, looking toward his family members sitting in the front row of the witness room.
"I'm going to go to the universe, and then I'll be back," he said. "God is here. The true god."
At one point, tears appeared to be rolling down his face.
Grant killed Brenda McElyea, 29, and Felicia Suzette Smith, 43, so there would be no witnesses to his robbery at the La Quinta Inn in Del City in July 2001.
"Although Donald Grant's execution does not bring Brenda back, it allows us all to finally move forward knowing that justice was served," Shirl Pilcher, a sister of McElyea, told reporters.
Grant was the first person executed in the U.S. this year and the third put to death in Oklahoma since the state resumed capital punishment after a six-year pause in October.
Grant and another death row inmate, Gilbert Postelle, had asked a federal judge to grant them a temporary injunction that would delay their executions until a trial can be held over whether Oklahoma’s three-drug lethal injection method is constitutional.
They had offered to be killed by firing squad as an alternative, arguing that it would be quicker and less painful.
Grant and other death row inmates in Oklahoma, as well as across the country, have challenged the use of the sedative midazolam in their executions, arguing that the drug is not appropriate for lethal injections and noting its use in several problematic executions.
In October, John Marion Grant heaved and vomited during his execution, causing members of the Oklahoma Pardon and Parole Board to question the execution process. However, the two executions since then, including Donald Grant's, were administered without issue.
A trial on the matter is set to begin Feb. 28. But Grant's scheduled execution was Thursday, and Postelle's date with death is set for Feb. 17.
The U.S. Supreme Court declined to stay Grant's execution Wednesday.