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, reports BBC.
They break with a 130-year-old precedent of pausing executions amid a presidential transition. Joe Biden becomes president on 20 January.
Death inmate's last words
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 the Supreme Court to halt it - but the request was denied.
The Brandon Bernard case
Bernard 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 back of their car in Texas.
They were shot in the car 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.
An independent investigator hired by the defence said Stacie was "medically dead" before the fire.
However, government testimony during the trial claimed that although Todd Bagley died instantly, Stacie had soot in her airway, signalling that she had died from smoke inhalation and not the gunshot wound.
Bernard's lawyers claim 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 has maintained a good record and worked with outreach programmes to stop people from getting involved in crime.