RENO, Nev. (AP) — The American Bar Association has joined an appeal challenging the death sentence of a Nevada man it says was mentally ill when he was convicted of the 1998 hatchet slaying of a campus police officer.

The national group of lawyers and professional legal scholars is asking the Nevada Supreme Court to block Siaosi Vanisi's (sigh-OH-see vuh-NEE-see) execution. It wants the justices to rule that people suffering from severe mental illness should never be sentenced to death.

Vanisi was found guilty of murdering University of Nevada, Reno police Sgt. George Sullivan as he was filling out paperwork in his cruiser on the campus just north of downtown. The native Tongan also was convicted of robbing a convenience store with Sullivan's service revolver before he was arrested during a standoff with a SWAT team in Salt Lake City.