The US government has carried out the first federal execution in almost two decades, putting to death a man who killed an Arkansas family in the 1990s.
The execution came over the objection of the victims' family.
Daniel Lewis Lee, 47, of Yukon, Oklahoma, died by lethal injection at the federal prison in Terre Haute, Indiana.
"I didn't do it," Lee said just before he was executed.
"I've made a lot of mistakes in my life, but I'm not a murderer... You're killing an innocent man."
The decision to move forward with the execution – the first by the Bureau of Prisons since 2003 – drew scrutiny from civil rights groups and the relatives of Lee's victims, who had sued to try to halt it, citing concerns about the coronavirus pandemic.
"The government has been trying to plough forward with these executions despite many unanswered questions about the legality of its new execution protocol," said Shawn Nolan, one of the lawyers for the men facing federal execution.
The developments are likely to add a new front to the national conversation about criminal justice reform in the lead-up to the 2020 elections.
One of Lee's lawyers, Ruth Friedman, said it was "shameful that the government saw fit to carry out this execution during a pandemic".
"And it is beyond shameful that the government, in the end, carried out this execution in haste," Ms Friedman said in a statement.
But US Attorney General William Barr said, "Lee finally faced the justice he deserved. 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."
The execution of Lee, who was pronounced dead at 8.07am local time, went off after a series of legal volleys that ended when the Supreme Court stepped in early on Tuesday in a 5-4 ruling and allowed it to move forward.
Mr Barr has said the Justice Department has a duty to carry out the sentences imposed by the courts, including the death penalty, and to bring a sense of closure to the victims and those in the communities where the killings happened.
But relatives of those killed by Lee in 1996 strongly opposed that idea and long argued that Lee deserved a sentence of life in prison. They wanted to be present to counter any contention that the execution was being done on their behalf.
"For us it is a matter of being there and saying, 'This is not being done in our name; we do not want this'," relative Monica Veillette said.
They noted that Lee's co-defendant and the reputed ringleader, Chevie Kehoe, received a life sentence.
Kehoe, of Colville, Washington, recruited Lee in 1995 to join his white supremacist organisation, known as the Aryan Peoples' Republic.
Two years later, they were arrested for the killings of gun dealer William Mueller, his wife, Nancy, and her eight-year-old daughter, Sarah Powell, in Tilly, Arkansas.
At their 1999 trial, prosecutors said Kehoe and Lee stole guns and 50,000 dollars in cash from the Muellers as part of their plan to establish a whites-only nation.
Prosecutors said Lee and Kehoe incapacitated the Muellers and questioned Sarah about where they could find money and ammunition.
Then, they used stun guns on the victims, sealed rubbish bags with duct tape on their heads to suffocate them, taped rocks to their bodies and dumped them in nearby water.
A US District Court judge put a hold on Lee's execution on Monday, over concerns from death row inmates on how executions were to be carried out, and an appeals court upheld it, but the high court overturned it.
That delay came after an appeals court on Sunday overturned a hold that had been put in place last week after the victims' relatives argued they would be put at high risk for the coronavirus if they had to travel to attend the execution.
Two other federal executions are scheduled for later this week, though one is on hold in a separate legal claim.
There have been two state executions in the US since the pandemic forced shutdowns nationwide in mid-March - one in Texas and one in Missouri, according to the Death Penalty Information Centre.
Alabama carried out one in early March.
Executions on the federal level have been rare, and the government has put to death only three defendants since restoring the federal death penalty in 1988 - most recently in 2003, when Louis Jones was executed for the 1995 kidnapping, rape and murder of a young female soldier.
Though there had not been a federal execution since 2003, the Justice Department has continued to approve death penalty prosecutions and federal courts have sentenced defendants to death.
In 2014, following a botched state execution in Oklahoma, President Barack Obama directed the Justice Department to conduct a broad review of capital punishment and issues surrounding lethal injection drugs.
The attorney general said last July that the Obama-era review had been completed, clearing the way for executions to resume.
He approved a new procedure for lethal injections that replaces the three-drug combination previously used in federal executions with one drug, pentobarbital. This is similar to the procedure used in several states, including Georgia, Missouri and Texas, but not all.
Numbers of state executions have fallen steadily since the last federal execution, according to data compiled by the Death Penalty Information Centre.
States put to death 59 people in 2004 and 22 in 2019, nine of which were in Texas.