Published: Oct 23, 2012 10:17 AM EDT

MIAMI (AP) - The execution of a convicted mass killer is back on after a federal appeals court lifted a stay put in place over the weekend by a district judge.
    
The stay was lifted late Monday by the U.S. 11th Circuit Court of Appeals in Atlanta, meaning 64-year-old John Ferguson could be executed as early as Tuesday evening.
    
A federal judge in South Florida on Saturday ruled that issues raised by Ferguson's lawyers should be explored further. His lawyers say Ferguson is mentally ill and the Constitution prohibits the state from executing him.
    
Ferguson was convicted of killing eight people including six who died execution-style in 1977 in Carol City. He was also convicted of killing a teenage couple in 1978.
    
Ferguson has been on death row for more than three decades.

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