OKLAHOMA CITY (AP) — An Oklahoma death row inmate who tried to delay his execution by challenging the state's lethal injection method was executed Tuesday evening just hours after the U.S. Supreme Court refused to step in.

Michael Hooper, convicted for the December 1993 shooting deaths of his former girlfriend and her two young children, received a lethal dose of drugs at the Oklahoma State Penitentiary in McAlester. The 40-year-old was pronounced dead at 6:14 p.m., according to the Department of Corrections.

Hooper was sentenced to death for killing 23-year-old Cynthia Lynn Jarman and her two children, 5-year-old Tonya and 3-year-old Timmy. Prosecutors said the three were with Hooper in a pickup truck in a mowed field when he placed a 9mm pistol under Cynthia Jarman's chin and shot her, then shot the children to prevent them from being witnesses.

Each of the victims was shot twice in the head, and their bodies were buried in a shallow grave in a field northwest of Oklahoma City.

Strapped to a hospital gurney with intravenous tubes inserted into each of his arms, Hooper spoke as his and his victims' relatives watched his execution through a window from separate rooms.

"I just want to thank God for such an exuberant send-off," he said as other death row inmates banged their cell doors in a tribute to the condemned man. "Also, my family for standing by me throughout all this. I appreciate them being there for me through the hardships."

Hooper did not directly speak to the victims' family members but indicated he sought their forgiveness.

"I ask that my spirit be released directly into the hands of Jesus. I'm ready to go," he said, then turned to family members, including his mother, brother and grandfather, and smiled as the drugs began to flow at 6:08 p.m.

"I love you all," he said, then deeply exhaled and closed his eyes. Minutes later, he lay motionless.

The victims' family later issued a statement offering "our sincerest condolences" to Hooper's relatives. "This has been a long and arduous journey for all of the families," the statement said. "We hope to close this chapter in our lives."

In an effort to at least stall his execution, Hooper had sued the state last month claiming Oklahoma's three-drug lethal injection protocol was unconstitutional. The lawsuit sought to force the state to have an extra dose of pentobarbital, a sedative, on hand during his execution.

Pentobarbital is the first drug administered and is used to render a condemned inmate unconscious. It's followed by vecuronium bromide, which stops the inmate's breathing, then potassium chloride to stop the heart.

Hooper's attorney, Jim Drummond, had argued that if the sedative were ineffective, the remaining drugs could cause great pain in violation of the Eighth Amendment's prohibition against cruel and unusual punishment. The lawsuit also noted that other states have adopted a one-drug process using a fast-acting barbiturate that supporters say causes no pain.

But the claims were rejected by a federal judge, then upheld by a federal appeals court. And the U.S. Supreme Court rejected Hooper's request without elaboration earlier Tuesday.

Hooper was the fourth death-row inmate executed in Oklahoma this year. Gary Roland Welch was executed Jan. 5 for fatally stabbing a 35-year-old man, and Timothy Stemple was executed on March 15 for the beating death of his wife. Michael Selsor was put to death on May 1 for the shooting death of a Tulsa convenience store manager.