Daniel Lewis Lee was killed by lethal injection over the objections of the victims’ family after the US Supreme Court in a late-night opinion ruled that the execution could go ahead.
Before he was put to death, Lee reiterated that he was an innocent man, saying, "I didn't do it. ... I've made a lot of mistakes in my life, but I'm not a murderer."
Lee was pronounced dead at 8:07 a.m. EDT, U.S. Bureau of Prisons spokeswoman Kristie Breshears said, as quoted by Reuters.
Lee was convicted in Arkansas of the 1996 killings of gun dealer William Mueller, his wife, Nancy, and her 8-year-old daughter, Sarah Powell. The victims' relatives were against the execution, saying that they wanted Lee to spend the rest of his life behind bars.
Several executions were initially delayed when US District Court in Washington ruled on Monday that there were still some unresolved legal challenges.
According to the Death Penalty Information Centre, there are 62 inmates currently on federal death row.
The Trump administration has been seeking to revive the government’s use of the punishment which has long been delayed due to legal issues.