A murderer mouthed 'I love you' and turned purple before he was put to death in Nebraska's first execution since 1997, and the first in the US to use fentanyl.

Carey Dean Moore, 60, was given the lethal injection with a deadly cocktail of drugs, including the synthetic opioid, on Tuesday morning at the Nebraska State Penitentiary in Lincoln.

The convicted killer, who had been on death row for 38 years, pronounced dead at 10.47am local time after being injected with a cocktail of four drugs, including the highly-addictive opioid.

He was sentenced to death for shooting and killing Omaha cab drivers, Maynard Helgeland and Reuel Van Ness Jr during a robbery in 1979.

When asked for his final words, Moore replied: 'Just the statement that I delivered to you about my brother Donny and the innocent people [on] Nebraska’s death row.'

Carey Dean Moore, 60, was executed by lethal injection in Nebraska at 10.47am on Tueaday for the 1979 murders of two Omaha cab drivers. It was the state's first execution since 1997 and the first in the US to use the fentanyl in the procedure

In his own words: Less than two weeks before his execution, Moore penned his final statement, in which he asked forgiveness from his younger brother Donald. He apologized for being a bad example to 'Donny', who he brought with him for the deadly robbery when he was just 14

He was referring to a letter he wrote on August 2, where he stated his desire for his younger brother, Donald Moore, to be taken off parole.

Moore expressed remorse for taking him along for the deadly robbery when he was just 14 years old and for setting a bad example.

The killer was seen mouthing 'I love you' several times to witnesses in the death chamber as the lethal drugs were administrated.

Moore then turned red and purple before drawing his last breath, according to witnesses inside the death chamber.

Until his death on Tuesday, he was the longest serving inmate on death row in the US.

The execution marked the first lethal injection in Nebraska, which last carried out the death penalty by using the electric chair 21 years ago.

The killer made headlines while behind bars in 1984, when he swapped places with his twin brother, who was incarcerated in the same prison for burglary, and got into general population.

Carey and his identical twin, David (pictured holding brother's photo), were both at Nebraska State Penitentiary when, in 1984, they swapped clothes after convincing guards to let them have a private family meeting. David went back to his brother's death row cell and Carey went to the kitchen

'As his older brother who him looked up to, I should had lead him in the right way to go in stead of bringing him down, way down, and because of that I am terribly sorry. (Please forgive me, Don, somehow),' he wrote.

The convicted killer also suggested that four other Nebraska death row inmates might be innocent.

'I am guilty, they are not!' Moore declared.

Moore penned the letter less than two weeks before his scheduled execution.

But he had already predicted his fate and signed it, 'Carey Dean Moore ex-Death Row Inmate.'

Moore had three witnesses and a clergyman watching his execution, but they have not been named.

In all, there were 10 people watching the execution, but none of the victims' family members were in attendance.

Moore went through seven execution dates while on death row before he was killed Tuesday.

His execution was a pivotal test for Nebraska, where the state legislature abolished the death penalty in 2015, only to see voters reinstate it the following year in a referendum

Nebraska Gov. Pete Ricketts, a Republican and a wealthy former businessman, helped finance a ballot drive to reinstate capital punishment after lawmakers overrode his veto .

His administration then changed Nebraska's lethal injection protocol to overcome challenges in purchasing the necessary drugs and withheld records previously considered public that would identify the state's supplier.

Last week, Moore's identical twin, David, said he was planning to watch his brother's execution. It is not known whether he attended.

The siblings made headlines in 1984 when they found themselves in the same prison and they swapped places for several hours before being caught.

David had been locked up for four to six years on burglary charges.

They convinced prison guards to let them have a private meeting to discuss a 'family matter' and changed clothes while no one was looking.

Afterwards, David was led to his brother's death row cell and Carey was taken to the kitchen, where David had a job.

The brothers' attempt to switch places was reported in 1984. Above is an article about their stunt which appeared in The New York Times

They were found out by the guard who was supervising the kitchen who knew that David was thinner than his brother.

The other clue was that Carey did not know where to put away the pots and pans.

At the time, prison guards did not know if the switch was a prank or if the brothers were plotting an escape.

They deduced that it was part of a grander escape plan but neither brother admitted to it once they were caught.

'Maybe Carey Dean wanted to get out of death row for awhile because it's pretty restrictive.

'Maybe he wanted to get out for awhile to break the boredom of death row and do something like pitch horseshoes,' John Dahm, assistant director of the state corrections department, told The Associated Press at the time.

Adding that it was the 'obvious conclusion' the pair were plotting an escape, he added: 'Both have been in trouble often with the law and I don't think they are considered funny or pranksters.'

The prison guard who caught the pair before they could carry out any such plan was Bob Austin.

He said he became suspicious when the heavier, 'ruddier' twin came back to the kitchen to work.

'Unless you saw them every day, you couldn't tell them apart. They're awfully close, but Carey Dean is probably 10 to 15 pounds heavier and he's got a ruddier complexion,' he said.

The other clue was that Carey did not know where to put the pots and pans.

'David would have known where to put the pan. But I probably wouldn't have picked it up if we hadn't had the other clues,' he said.

In 1979, Carey Dean murdered cab drivers Maynard Helgeland (left) and Reuel Van Ness (right) within days of each other and robbed both men

Their punishment was a temporary ban on seeing each other in the prison.

David was later released and his brother was sent to the Tecumseh State Correctional Institution.

The pair had grown up poor in a family of 13 and turned to crime to get by as teenagers.

'Unfortunately, we didn't learn to respect people before something really bad happened,' David said in the run-up to his brother's execution. 'Back then we were both animals. We weren't fit to be allowed in society, I guess.'

In July, his execution date was finally set for August 14.

The drug used to kill him were set to expire in a few weeks.

A last-minute lawsuit from German pharmaceutical company Fresenius Kabi also failed to stop the execution.

Carey Dean Moore is pictured in a 2007 prison photograph. He was in jail from 1979 until his execution on Tuesday

The company accused state officials of improperly using its drugs, but a judge said he wouldn't delay the execution.

The state noted that one of its execution drugs was set to expire on August 31 and that prison officials wouldn't be able to purchase more.

Nebraska lawmakers abolished capital punishment in 2015 by narrowly voting to override Governor Ricketts' veto of the legislation.

Some legislators expressed doubt at the time that Nebraska would carry out an execution ever again because of costly legal challenges.

That prompted Ricketts to ask for more time to set one in motion.

He eventually contributed $300,000 of his own money to a petition drive organized by several close associates to place the issue on the November 2016 general election ballot.

The governor's father, TD Ameritrade founder Joe Ricketts, also donated $100,000 to the effort.

There are now 10 death row prisoners in Nebraska - all of whom are murderers. No other executions have been scheduled for 2018.