Milwaukee County Circuit Judge Jeffrey Wagner requested a pool of 50 potential jurors as jury selection began, a larger than usual number owing to the publicity the case has received.
Spooner, 76, is charged with first-degree intentional homicide after allegedly shooting Darius Simmons in May, while Simmons' mother watched.
Spooner suspected Simmons of stealing guns from his home, confronted the teen outside his home and shot Simmons in the chest from about five feet away, the criminal complaint states.
The trial was scheduled for May and delayed due to health concerns of his attorney, then postponed again in May when an expert defense witness broke his arm and was unable to attend the trial. Monday's trial began late as cases backlogged by a courthouse fire last week took up most of the morning, the Milwaukee Journal Sentinel reported.