* John Errol Ferguson was scheduled to be executed on Tuesday
* He has been on death row for 34 years
* His lawyers contend he is mentally insane
The U.S. Supreme Court on Tuesday upheld a last-minute federal appeals court ruling granting a stay of execution to a mentally ill Florida man convicted of killing eight people, including two teenagers, during the 1970s.
John Errol Ferguson, 64, had been scheduled to be executed around 6 p.m. EDT (2200 GMT) but his lawyers filed a flurry of appeals late Tuesday arguing he should not be put to death because he is mentally insane.
The ruling by the 11th U.S. Circuit of Appeals establishes a schedule for additional briefs to be filed in Ferguson's case by Nov. 6.
Ferguson, who has been on death row for 34 years, was convicted of first-degree murder and sentenced to death in 1978 for a pair of killing sprees.
In July 1977, he fatally shot six people execution-style during a drug-related home robbery in a northern Miami suburb. Six months later, he killed two teenagers after they left a church meeting.
State psychiatrists and other medical professionals have diagnosed Ferguson as a paranoid schizophrenic with a long history of mental illness, according to his defense team.
Defense attorney Christopher Handman has said Ferguson considers himself the "Prince of God" and does not understand the death penalty and why it had been imposed.
"It is impossible to fathom that the state can constitutionally put to death a man who thinks he is the 'Prince of God' and who believes he has a destiny of being the right hand of God and returning to purify earth after the state tries to kill him," Handman said in a statement earlier this month.
Courts, however, have repeatedly rejected the claims.
On Monday, a federal appeals court rejected a new attempt by Ferguson's lawyers to postpone the execution, overturning an order by a south Florida U.S. District Court judge that the execution be delayed.
In early September, Florida Governor Rick Scott signed Ferguson's death warrant, but a few weeks later delayed the execution while a team of physicians met to decide whether Ferguson is mentally competent.
After a 90-minute examination and brief consultation earlier this month, a panel of psychiatrists determined that Ferguson was sane. A state circuit judge agreed in a ruling.
Posing as a utility company worker, Ferguson entered the home of Margaret Wooden, 24, on July 27, 1977 in the Miami suburb of Carol City. He pulled a gun, tied her up and called two accomplices into the home to search for drugs and money.
Six of Wooden's friends arrived at the house soon after and were tied up. Her boyfriend, Michael Miller, 24, also turned up. Six of the victims, including Miller, were shot to death. Wooden and another person escaped.
Ferguson was also found guilty of the abduction and killing of two 17-year-old Miami high school students.
According to court records, Ferguson abducted Belinda Worley and her boyfriend, Brian Glenfeldt, after they left a social event at a church on Jan. 8, 1978.
Their bodies were found the next day in a nearby wooded area. Glenfeldt was shot in the head, chest and arm. Worley was found in dense brush with a gunshot to the back of the head. An autopsy revealed she had been raped.