Daniel Lewis Lee, 47, of Yukon, Oklahoma, died by lethal injection this week at the federal prison in Terre Haute, Indiana.
Lee, a self-confessed white supremacist, was convicted in Arkansas of the 1996 killings of gun dealer William Mueller, his wife Nancy, and her 8-year-old daughter, Sarah Powell.
'I didn't do it,' Lee said just moments before he was executed at 8.07am EDT.
'I've made a lot of mistakes in my life, but I'm not a murderer... You're killing an innocent man.'
His execution, which came over the objection of the victims' family, was carried out after a series of legal volleys that ended when the Supreme Court stepped in early Tuesday in a 5-4 ruling and allowed it to move forward.
The decision to move forward with the execution - the first by the Bureau of Prisons since 2003 - has drawn scrutiny from civil rights groups and the relatives of Lee's victims, who had sued to try to halt it.
Critics have argued that the Trump administration, which has been pushing for the executions, was creating an unnecessary and manufactured urgency for political gain.
Two other federal inmates are also scheduled to be executed later this week and a third next month.
'The government has been trying to plow forward with these executions despite many unanswered questions about the legality of its new execution protocol,' said Shawn Nolan, one of the attorneys 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.
Lee's execution was scheduled for Monday afternoon but was put on hold just hours earlier by a US District Court judge over concerns from death row inmates on how executions were to be carried out.
An appeals court upheld the decision, 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.
Lee's execution was then set to happen at 4am EDT, but a last-minute legal question was raised by his lawyers. The Justice Department said in a statement it filed a request with the court to straighten it out but went through with the execution.
A US Marshal lifted a black telephone inside the execution room - a small square room inside the prison with green tiles and windows looking at the witness rooms - and asked if there was anything to impede the execution.
He said there was not and the execution could proceed.
Lee had a pulse oximeter on a finger of his left hand and his arms were in black restraints. The IV tubes were coming through a metal panel in the wall.
He breathed heavily before the drug was injected and moved his legs and feet. As the drug was being administered, he raised his head to look around. In a few moments, his chest was no longer moving.
Lee was in the execution chamber with two men who the Bureau of Prisons would only identify as 'senior BOP officials,' a US Marshal and his spiritual adviser, who a Bureau of Prisons spokesperson described as an 'Appalachian pagan minister'.
They did not wear masks and Lee was also not wearing a mask.
Attorney General William 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 but argued they would be put at high risk for COVID-19 if they attended.
'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.
The relatives 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 orgaization, 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 8-year-old daughter, Sarah Powell, in Tilly, Arkansas, about 75 miles northwest of Little Rock.
Lee and Kehoe dressed in police raid clothing and lay in wait for Mueller in his home the night of the murders, according to court documents.
When the Muellers returned home, Lee and Kehoe overpowered and incapacitated Mueller and his wife.
They then interrogated the couple's young daughter, Sarah, about where they could find cash, guns, and munitions.
The home invaders found and took roughly $50,000 in cash, guns and ammunition.
After robbing and torturing the victims with a stun gun, prosecutors said Lee covered their heads with plastic bags, sealed the bags with duct tape, weighed down each victim with rocks, and threw the family of three into the Illinois Bayou.
The bodies of the three victims were found five months after they went missing.
