Nebraska has become the first US state to carry out an execution with the use of the powerful opioid fentanyl.

Carey Dean Moore, 60, was pronounced dead at 10.47am local time on Tuesday after being sentenced to death for killing taxi cab drivers in Omaha in 1979.

He was the first inmate to be lethally injected in Nebraska, which last carried out an execution in 1997, using the electric chair.

Carey Dean Moore
Carey Dean Moore was executed with a lethal injection (Nebraska Department of Correctional Services via AP, File)

Witnesses said that there appeared to be no complications in the execution process, which was also the first time a state used four drugs in combination. Moore remained mostly still throughout the execution but breathed heavily and gradually turned red and then purple as the drugs were administered.

Media witnesses saw Moore take short, gasping breaths that became deeper and more laboured. His chest heaved several times before it went still. His eyelids briefly cracked open.

At one point while on the gurney, Moore turned his head and mouthed several words to his family, including “I love you”. No members of the victims’ families witnessed the execution.

The Department of Correctional Services said the first lethal injection drug was administered at 10.24am.

Protesters outside the Nebraska State Penitentiary
Protesters outside the Nebraska State Penitentiary (Eric Gregory /Lincoln Journal Star via AP)

In his final written statement, Moore admitted: “I am guilty.”

But he said there are others on Nebraska’s death row who he believes are innocent and he said they should be released.

“How might you feel if your loved one was innocent and on death row?” Moore asked.

Moore’s execution comes a little more than three years after Nebraska abolished the death penalty, only to have it reinstated the following year through a citizen ballot drive partially financed by Republican governor Pete Ricketts.

The governor, a wealthy former businessman, has said he was fulfilling the wishes of voters in the conservative state.

The Nebraska drug protocol called for an initial IV dose of diazepam, commonly known as Valium, to render the inmate unconscious, followed by the powerful synthetic opioid fentanyl, then cisatracurium besylate to induce paralysis and stop the inmate from breathing and potassium chloride to stop the heart.

A man kneels outside the Nebraska State Penitentiary where careyy was executed
A man kneels outside the Nebraska State Penitentiary where careyy was executed (Eric Gregory /Lincoln Journal Star via AP)

Diazepam and cisatracurium had also never been used in executions before.

According to prosecutors, Moore was 21 when he shot dead Reuel Van Ness during a robbery with his younger brother, and used the money to buy drugs and pornography.

Moore fatally shot Maynard Helgeland by himself five days later, saying he wanted to prove he could take a man’s life by himself.

Moore was arrested a week later. He was charged and convicted of first-degree murder, while his 14-year-old brother was convicted of second-degree murder.

In his statement, Moore also apologised to his brother for dragging him into the robbery and murder.

“I should (have) led him in the right way to go instead of bringing him down, way down,” Moore said.