Nicklasson was pronounced dead just before 11 p.m., KSDK-TV reported. He had been scheduled to die just after midnight but a federal appeals court stayed the execution late Tuesday.
Missouri Attorney General Chris Koster, who appealed to the Supreme Court, welcomed the decision and spoke of the pain endured by the family of Nicklasson's victim, Richard Drummond.
"The highest court in the nation has removed the last restriction to carrying out the lawfully imposed punishment of Allen Nicklasson," Koster said in a statement. "While the sentence carried out this evening cannot lessen the loss for Mr. Drummond's family, it nonetheless will give them the knowledge that justice has been done. My thoughts and prayers tonight are with the family and friends of Mr. Drummond."
The execution was Missouri's second within a few weeks after a three-year hiatus.
Nicklasson was sentenced to death in 1996 for the murder of Drummond, an AT&T executive who stopped to help after his vehicle broke down on I-70. A co-defendant, Dennis Skillicorn, was put to death in 2009 for the killing, while Tim DeGraffenreid, who was 17 in 1994, is in prison.
Nicklasson and Skillicorn drove Drummond's car to Arizona, where they killed a couple who offered assistance after a breakdown. They received life sentences there.
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