The US Supreme Court has paved the way for President Donald Trump's administration to carry out the first federal executions since 2003, turning away an appeal by four inmates challenging the lethal injection protocols due to be used.
The justices left in place a lower court ruling that had let the executions proceed.
The men, convicted in federal courts of murder, had appealed after the US Court of Appeals for the District of Columbia Circuit on 7 April threw out a judge's injunction that had blocked the executions.
The inmates - Daniel Lee, Wesley Purkey, Alfred Bourgeois and Dustin Honken - are scheduled for execution in July and August at a federal prison in Terre Haute, Indiana.
The brief court order noted that two of the four liberal justices, Ruth Bader Ginsburg and Sonia Sotomayor, favoured hearing the appeal.
Lee, a white supremacist, is due to be the first executed, on 13 July. He was convicted in Arkansas in the suffocation deaths carried out with an accomplice of a gun dealer, the man's wife and her eight-year-old daughter in 1996.
While some individual states have continued to carry out executions of inmates convicted in their courts, the US government has not conducted an execution since 2003 during the administration of President George W Bush amid protracted litigation over the practice.
But Attorney General William Barr last year announced that the Trump administration would resume executions, citing the need to uphold "the rule of law" and saying that "we owe it to the victims and their families to carry forward the sentence imposed by our justice system."
At issue in the legal battle is whether a US law called the Federal Death Penalty Act requires the federal government to follow all execution protocols in the state where the sentence was imposed as opposed to where the execution will take place.
The law requires that the execution be implemented "in the manner prescribed by the law of the state in which the sentence is imposed," which the federal government has interpreted loosely as the general method of execution, which in all four cases would be lethal injection.
But lawyers for the inmates have argued that under this law the government must follow the detailed execution protocols in the states where the inmates were convicted.
Lee was convicted in Arkansas, Purkey in Missouri, Bourgeois in Texas and Honken in Iowa.
Iowa is among the states that does not have the death penalty, but Honken was sentenced to death, as federal law allows, subject to Indiana's method of execution.