The US government has carried out its first execution in 17 years, putting to death convicted murderer Daniel Lee after the Supreme Court cleared the way with an overnight ruling.
Lee was pronounced dead at 8.07am local time on Tuesday, a US Bureau of Prisons spokeswoman said.
His death marked the culmination of a three-year effort by US President Donald Trump's administration to resume capital punishment.
It ended a de facto moratorium by Trump's predecessor Barack Obama, amid legal challenges and difficulties obtaining lethal-injection drugs.
US Attorney General William Barr endorsed the execution in a statement.
"The American people have made the considered choice to permit capital punishment for the most egregious federal crimes, and justice was done today in implementing the sentence for Lee's horrific offences," he said.
The execution by lethal injection in Terre Haute, Indiana, was preceded by a flurry of legal challenges in multiple federal courts.
At 2.10am local time, the Supreme Court in a 5-4 vote cleared the way for federal executions to resume, ruling that Lee and other condemned men's challenges to the execution protocol did not justify "last-minute" intervention by federal courts.
Ruth Friedman, one of the public defenders who represented Lee, rebuked the Department of Justice for what she described as a rushed process.
In a statement, she said it was "beyond shameful that the government, in the end, carried out this execution in haste, in the middle of the night, while the country was sleeping".
"We hope that upon awakening, the country will be as outraged as we are," she said.
Lee was convicted for his role in the killing of three members of an Arkansas family in 1996.
But some relatives of his victims opposed him receiving the death sentence, while his accomplice in the murders, Chevie Kehoe, was sentenced to life in prison.
The victims' relatives unsuccessfully sued last week to delay the execution until the coronavirus pandemic had passed, saying they feared attending would risk their safety.
Strapped to a gurney before his death, Lee was asked if had any last words, according to a media witness in the viewing chamber.
"I didn't do it. I've made a lot of mistakes in my life but I'm not a murderer," Lee said, according to the media witness, who issued a report for other news outlets.
"You're killing an innocent man."
As the drug was being administered, Lee raised his head to look around, and his breathing appeared to become laboured, according to the pool report.
Soon after, Lee's chest was no longer moving, his lips turned blue and his fingers became ashy.
Two unnamed Bureau of Prisons officials and Lee's spiritual adviser could be seen inside the execution chamber.
The Justice Department has scheduled two more executions of convicted child killers this week.
Australian Associated Press