Oklahoma executed a death row inmate by lethal injection on Thursday morning. The inmate had been convicted of killing his ex-girlfriend and her baby almost two decades ago. 

Raymond Johnson, 52, was pronounced dead at 10:12 a.m. Thursday following a three-drug injection at the Oklahoma State Penitentiary in McAlester, prison officials said.

He was sentenced to death for killing 24-year-old Brooke Whitaker and her 7-month-old daughter, Kya, in June 2007.

Johnson was the 11th person executed in the United States so far this year, and the second executed in Oklahoma. Another Oklahoma inmate, Kendrick Simpson, received a lethal injection in February for the drive-by shooting deaths of two men in 2006. Florida has carried out five executions since January, more executions than any other state.

Prosecutors said Johnson and Whitaker had been arguing at her home in Tulsa before he repeatedly hit her over the head with a metal claw hammer. Whitaker's skull was fractured and she had more than 20 lacerations on her face and scalp. But she was still conscious and begged Johnson to spare her and Kya, who was sleeping in a bedroom, prosecutors said in documents prepared for Johnson's clemency hearing in April.

"She begged him to call 911. She begged him to let her mom come get baby Kya. She begged him to think of her children," the Oklahoma Attorney General's Office said. Whitaker had three other children.

Johnson retrieved a gas can from a tool shed in the backyard, doused Whitaker and the house with gasoline, lit a dish towel on fire, threw it at Whitaker and left, the attorney general's office said. Whitaker died from head injuries and smoke inhalation while her daughter died from severe burns.

"Raymond Johnson is a cruel murderer who inflicted unimaginable pain and suffering on his victims," Oklahoma Attorney General Gentner Drummond said in a statement.

Johnson's attorneys did not file a last-minute appeal with the U.S. Supreme Court to stop his execution.

His attorneys unsuccessfully argued in earlier appeals that Johnson's arrest was illegal, police coerced a confession from him and Johnson's trial lawyer conceded his guilt in Whitaker's death without his permission.

In April, Oklahoma's five-member Pardon and Parole Board voted unanimously to deny Johnson clemency. During that clemency hearing, Johnson apologized to the victims' family and asked for forgiveness, saying he was a changed person.

"I apologize. No excuses, no justifications, a sincere apology. And to know that it's sincere, look at my actions. Look at my life. Look how I've changed. I'm living a remorseful life. I'm living it," Johnson said in an interview with Death Penalty Action, a national anti-death penalty group.

Whitaker's family members asked for the lethal injection to proceed.

"Executing him will not give me my mom or sister back, it will not take away almost 20 years of pain. What it will do is finally stop him from continuing to hurt us," Logan Kleck, Whitaker's oldest daughter, said in a letter to the board.

In addition to his first-degree murder conviction, Johnson also served nine years of a 20-year sentence after being convicted of manslaughter in 1996.