A death row inmate has been executed for killing his wife and stepsons before raping his stepdaughters.

Robert Sparks, 45, was given a lethal injection at 6pm (2300 GMT) on Wednesday at a death chamber in Huntsville, Texas, for the 2007 murders.

A jury convicted Sparks of capital murder in 2008 after he stabbed his 30-year-old wife Chare Agnew 18 times as she lay in bed in their Dallas home.

He then went into the bedroom of nine-year-old Harold Sublet and 10-year-old Raekwon Agnew and took them into the kitchen separately, where he stabbed them.

Raekwon was stabbed at least 45 times.

In Pictures: Capital punishment in Texas Show all 6 1 /6 In Pictures: Capital punishment in Texas In Pictures: Capital punishment in Texas 'Old Sparky', the decommissioned electric chair in which 361 prisoners were executed between 1924 and 1964, at the Texas Prison Museum in Huntsville, Texas. AFP AFP In Pictures: Capital punishment in Texas The Polunsky Unit, where death row inmates are held, in Livingston, Texas, about 40 miles from Huntsville. AFP AFP In Pictures: Capital punishment in Texas A holding cell for death row inmates that are scheduled to be executed in the Texas death chamber in Huntsville. Hulton Archive Hulton Archive In Pictures: Capital punishment in Texas A bed used for administering the lethal injection to death row prisoners in Huntsville, Texas. Getty Images In Pictures: Capital punishment in Texas The front door to a Texas death chamber in Huntsville. Getty Images In Pictures: Capital punishment in Texas A cemetery for prisoners, with some three thousand graves since the 19th century in Huntsville, Texas. Last year, Texas executed its 500 convict since the death penalty was restored in 1976, a record in a country where capital punishment is elsewhere in decline. AFP AFP

Sparks then raped his 12- and 14-year-old stepdaughters, authorities said.

He locked his stepdaughters in a cupboard and left the home, stopping at his mother’s house to borrow her car, and then his ex-girlfriend’s home, where he told her he had killed his wife and two stepsons.

On arrest, he told police his wife had been poisoning him.

He provided blood, hair and fingernail samples, as well as a cheek swab, to be tested for evidence of poisoning, but investigators were unable to find a lab capable of that type of test, according to court papers.

He also told a psychologist that a voice told him “to kill them because they were trying to kill me”.

As relatives and friends watched through a death chamber window, Sparks told them: “I am sorry for the hard times. And what hurts me is that I hurt y’all.”

Speaking after his death, one of Sparks’ stepdaughters said: “The day when the situation was going on, he said that we wouldn't make it.

“Twelve years later, we’re both standing here... I want him to know we’re not suffering. We’re hurt emotionally but physically we’re fine.”

Sparks’ lawyers had argued he suffered from severe mental illness and had been diagnosed as a delusional psychotic and with schizoaffective disorder.

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They also claimed he was intellectually disabled and was therefore not eligible for the death penalty, however the 5th US Circuit of Appeals on Tuesday declined to stop the execution on these grounds.

Sparks also argued that a prosecution expert gave false testimony and that the trial jury may have been influenced by a bailiff who wore a tie with an image of a syringe which showed his support for the death penalty.

Sparks is the 16th inmate in the United States and the seventh in Texas to be executed in 2019, according to the Death Penalty Information Centre.

Texas has executed more prisoners than any other state since the Supreme Court reinstated capital punishment in 1976.