Death row inmate Brandon Bernard has been executed in Indiana after last-minute clemency pleas were rejected by the US Supreme Court.
Bernard, 40, was convicted of murder in 1999 when he was a teenager, and is the youngest offender to be executed by the federal government in nearly 70 years.
Bernard told the family of the couple he killed he was sorry, before dying by lethal injection on Thursday.
Four more executions are planned before the end of Donald Trump’s presidency.
If all five take place, Mr Trump will have overseen the most executions by a US president in more than a century. It would bring to 13 the total of federal executions since July. They break with an 130-year-old precedent of pausing executions amid a presidential transition. Joe Biden becomes president on 20 January. Bernard was pronounced dead at 21:27 local time on Thursday (02:27 GMT Friday) at a penitentiary in the city of Terre Haute. Before that he directed his last words to the victims’ family, speaking calmly for more than three minutes.
“I’m sorry. That’s the only words that I can say that completely capture how I feel now and how I felt that day,” he said, according to the Associated Press.
The execution was delayed for more than two hours after Bernard’s lawyers asked in vain for the Supreme Court to halt it.
He was given the death penalty for his involvement in the murder of Todd and Stacie Bagley in June 1999.
He was one of five teenagers accused of robbing the pair and forcing them into the boot of their car in Texas.
They were shot as they lay in the boot by 19-year-old accomplice Christopher Vialva before Bernard set the car alight.
Defence lawyers say both of the Bagleys probably died before the car was set on fire, and an independent investigator hired by the defence said Stacie had been “medically dead” before the fire.
However, government testimony during the trial said that although Todd Bagley had died instantly, Stacie had had soot in her airway, signalling that she had died from smoke inhalation and not the gunshot wound.
Bernard’s lawyers say he feared what would happen to him if he refused to follow the orders of Vialva, who was executed in September.
Others involved in the incident were given prison sentences as they were under 18 and classed as juveniles.
Bernard’s lawyers argued that he should be given life in prison without parole, as, throughout his time in jail, he maintained a good record and worked with outreach programmes to stop people from getting involved in crime.
The federal prosecutor who defended Bernard’s death sentence had called for Bernard to see out his sentence in prison.
In an opinion piece published in the Indianapolis Star, Angela Moore wrote: “Having learned so much since 2000 about the maturation of the human brain and having seen Brandon grow into a humble, remorseful adult fully capable of living peacefully in prison, how can we say he is among that tiny group of offenders who must be put to death?”
Five of the surviving nine jurors called for Mr Trump to commute Bernard’s death sentence.
Tens of thousands of other people urged the president to grant Bernard clemency, including senators Richard J Durbin and Cory Brooker.
On Thursday, leading lawyers Allen Dershowitz and Ken Starr also joined his defence team.
Reality television star Kim Kardashian West sent numerous tweets about Bernard’s case to her followers in the run-up to the execution, urging them to get behind the cause and raise awareness.
Ms Kardashian West is studying to be a lawyer in California and has previously helped sway criminal cases.
In March she visited the White House with three women whose jail terms were cut by President Trump.
