A US district judge has ordered a new delay in federal executions, hours before the first lethal injection was scheduled to be carried out at a federal prison in Indiana.
The Trump administration immediately appealed to a higher court, asking that the executions move forward.
Late on Monday, the federal appeals court in Washington, DC, refused to allow the executions to proceed. The administration has asked the Supreme Court to step in.
US District Judge Tanya Chutkan said there were still legal issues to resolve and "the public is not served by short-circuiting legitimate judicial process".
The executions, pushed by the administration, would be the first carried out at the federal level since 2003.
Chutkan said the inmates have presented evidence showing that the government's plan to use only pentobarbital to carry out the executions "poses an unconstitutionally significant risk of serious pain".
Chutkan said the inmates produced evidence that, in other executions, prisoners who were given pentobarbital suffered "flash pulmonary edema", which she said interfered with breathing and produced sensations of drowning and strangulation.
The inmates have identified alternatives, including the use of an opioid or anti-anxiety drug at the start of the procedure or a different method altogether, a firing squad, Chutkan said.
The Justice Department appealed to the US Court of Appeals for the District of Columbia Circuit and the Bureau of Prisons continued with preparations in order to move forward the execution of Daniel Lewis Lee should the stay be lifted.
The new hold came a day after a federal appeals court lifted a hold on the execution of Lee, of Yukon, Oklahoma, which was scheduled for 4pm on Monday at the federal prison in Terre Haute, Indiana.
He was convicted in Arkansas of the 1996 killings of gun dealer William Mueller, his wife, Nancy, and her eight-year-old daughter, Sarah Powell.
Australian Associated Press