The US Supreme Court ruled on Tuesday that the first federal executions in 17 years could proceed, overturning an injunction blocking them in order to allow legal challenges to the government's lethal-injection protocol to continue.
Judge Tanya Chutkan of the US district court in Washington had on Monday ordered the justice department to delay four executions scheduled for July and August.
Chutkan's order was issued less than seven hours before the execution of Daniel Lee was due to take place in Terre Haute, Indiana. The order was later affirmed by a US appeals court.
"The plaintiffs in this case have not made the showing required to justify last-minute intervention by a Federal Court. Last-minute stays like that issued this morning should be the extreme exception, not the norm," the Supreme Court said.
"The government has produced competing expert testimony of its own, indicating that any pulmonary edema occurs only after the prisoner has died or been rendered fully insensate," the court added.
The authorities were prepared to move forward with Lee's execution at 4 am EDT (0800 GMT) on Tuesday after the Supreme Court's decision, documents filed in the district court in Washington by Lee's lawyers showed.
"In light of the imminent and irreparable harm that he faces, Lee respectfully requests that the court immediately issue a ruling on his pending supplemental claim and motion for preliminary injunction," his lawyers said.
Reuters could not verify if the execution took place at 4 am EDT. The US justice department did not immediately respond to requests for comment.
Attorney General William Barr had announced last July that the Justice Department would resume carrying out executions of some of the 62 inmates on federal death row.
He originally scheduled five executions for last December, but was ordered to delay them by Chutkan while long-running lawsuits challenging the government's lethal-injection protocol played out.
An appeals court overturned that injunction in April, and Barr announced new execution dates for July and August of four inmates, all men convicted of murdering children: Lee, Wesley Purkey, Dustin Honken and Keith Nelson.