ATLANTA (AP) — The man accused of a courthouse shooting rampage that left four people dead pleaded not guilty by reason of insanity as his trial began amid high security Thursday, more than three years after prosecutors say he confessed in the killings.

Defense attorney Henderson Hill entered the plea for Brian Nichols a day after filing a motion claiming that Nichols was insane and couldn't tell "right from wrong" during the killings.

Even Nichols' defense team has conceded he killed a judge, court reporter and sheriff's deputy at the Fulton County Courthouse in downtown Atlanta on March 11, 2005, and a federal agent later that day.