The Oklahoma execution of Michael Hooper, who was sentenced to death in 2004 for the triple murder of his girlfriend, and her two children, went ahead on Tuesday evening despite the inmate's attempt to delay. Hooper was the 100th inmate to be put to death in Oklahoma, since the state renewed capital punishment in 1990.

Hooper, 40, who was sentenced to death in 2004, requested a stay on his execution on the legal grounds that Oklahoma's method of lethal injection is unconstitutional. Oklahoma is one of many states that currently uses a 3-drug cocktail to lethally inject prisoners. The mixture is made up of a combination of pentobarbital, a barbiturate used to sedate the inmate, vecuronium bromide, which stops breathing, and potassium chloride, which stops the heart.

Last month, Hooper challenged that the state should have an additional dose of pentobarbital available on hand as a precautionary measure if the drug proved ineffective at sedating him. His attorney, Jim Drummond, argued that in the event that the sedative did not work, the latter drugs could cause him pain, a violation of the Eighth Amendment.

But a federal appeals court dismissed the claim, and Hooper was put to death hours after the U.S. Supreme Court refused to intervene in the case. The U.S. Court of Appeals for the Tenth Circuit said that Hooper "failed to offer anything more than speculation that the lack of a backup dose of pentobarbital would be dangerous."

Hooper was convicted of shooting his girlfriend, Cynthia Lynn Jarman, a 23-year-old cosmetologist, along with her two children and burying them in a shallow grave, in a field northwest of Oklahoma City.

The Associated Press reported that "Prosecutors alleged the victims were with Hooper in a pickup truck in a mowed field when Hooper placed the muzzle of a 9mm pistol under the chin of his former girlfriend, Cynthia, and shot her. Blood in her lungs indicated she had time to draw a partial breath before he shot her execution-style in the right temple, authorities said."

Diane Roggy, Jarman's mother, and grandmother of 5-year-old Tonya and 3-year-old Timmy, said that the execution was just but that it would not bring her closure. "It's not going to change what happened. But justice will be served," said Roggy. "It will never be over, in my mind, until they close my casket."

Jarman's family said that they had forgiven Hooper although he never publicly accepted responsibility for the three murders. Roggy and Jarman's sister, Cynthia Weber, said that when they visited Hooper at the Oklahoma State Penitentiary 8 years ago, he privately admitted killing them and apologized.

Hooper was pronounced dead at 6:14 pm on Tuesday.