The 21-year-old suspect in theshowed no emotion during his virtual court appearance on Wednesday.
Investigators saytook aim and fired 83 rounds at spectators from a rooftop, stopping to reload his rifle. Then he made a run for it in women's clothing, leaving his gun behind.
"He indiscriminately fired at the crowd of people and struck people at random," Highland Park Police chief Lou Jogmen told CBS News in an exclusive interview.
It was an attack he'd allegedly been planning for weeks.
"There was quite a bit of preplanning that went into it and he was quite motivated to carry out the attack," Jogmen said.
"My instant thought was this person could potentially get away and not be held accountable for this," Jogmen said. "And that concern stayed with me throughout the first couple of hours because we had such a poor description. Not a good trail."
Finding the gun is what "set this investigation on a totally different trajectory," Jogmen said.
"In this case, we knew the make, the model, the serial number, and then we went through with the tracing process directly to the manufacturer," ATF special agent in charge Kristen de Tineo told CBS News. All of that information allowed the ATF to identify the suspect.
During the frantic eight-hour manhunt, police say the suspect stopped to see an acquaintance and then drove two and a half hours to Madison, Wisconsin, where he contemplated carrying out another attack when he spotted a large gathering. In the car with him was another gun and approximately 60 rounds of ammunition. CBS News obtained a photo of the gun that was in the car.
Jogmen said investigators don't yet have a motive.
"We really don't have any better understanding today than we did when we first started talking to him about the why in particular," he said.
Police said the suspect confessed to the shooting that killed at least seven people and wounded 38 others. He's being held without bond andare expected to be filed against him in the coming weeks.
The Illinois State Police say the alleged gunman passed four background checks in 2020 and 2021 as he amassed an arsenal of five firearms. The first one was purchased after the suspect's father signed an authorization form.
Despite two encounters with police in 2019 involving threats of violence to himself or family members and a series of troubling social media posts, his parents' attorney says they saw no warning signs.
"To them, he was just their son," attorney Steve Greenberg told CBS News. "He was a little bit eccentric. He was into music. He was into art. But to them, he was just their son and there weren't really any red flags. It's a terrible tragedy for everybody."