Brandon Bernard has been executed in Indiana after last-minute clemency pleas were rejected by the Supreme Court.
The 40-year-old death row inmate 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.
Brandon Bernard told the family of the couple he killed he was sorry, before dying by lethal injection on December 10.
Four more executions are planned before the end of Donald Trump’s presidency.
If all five take place, President 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 January 20.
Brandon Bernard was pronounced dead at 21:27 local time on December 10 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.
According to the Associated Press, he said: “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.”
The execution was delayed for more than two hours after Brandon 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.
Brandon Bernard 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 Brandon Bernard set the car alight.
His 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.
Brandon 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 programs to stop people from getting involved in crime.