Fortunately, the party-goers were saved by a man with a concealed-carry permit who fired back at Gonnigan, wounding him and buying time for the police to get to the scene.
Gonnigan fired his gun into the air several times off of a nearby porch, prompting the guests at a small party to ask him to stop.
Children were present at the party.
The request angered Gonnigan, who emerged from his apartment building several minutes later and opened fire on the party-goers.
He first shot a 32-year-old man; then, as the party scattered, he fired a shot that grazed the abdomen of a 50-year-old man.
The 49-year-old man with a concealed carry permit then pulled out a handgun, returning fire and injuring Gonnigan.
He hit Gonnigan in both arms and the hip, causing him to retreat momentarily.
After Gonnigan fled, the group tried to tend to the wounded. But he soon returned and shot a woman in the face, killing her instantly.
Next, he stood over the wounded 32-year-old man and firing several more rounds into him.
The 32-year-old man suffered 10 gunshot wounds and is currently in critical condition at a Chicago-area hospital.
Gonnigan also shot the 50-year-old two more times before returning to his apartment.
The police soon arrived and arrested Gonnigan, who has previously received a 10-year prison sentence for aggravated assault.
He has been charged with one count of murder, two counts of attempted murder, and one count of aggravated assault.