A portrait of Robert “Bobby” E. Crimo III as a mysterious and music-obsessed loner began emerging Tuesday, a day after he was arrested in the wake of a mass shooting at a July Fourth parade that left seven dead and traumatized an affluent Chicago suburb.

Crimo, 21, who dressed in women’s clothing during the deadly rampage, aspired to be a rapper and his music got darker and more delusional as he got older and relationships with his parents and a girlfriend frayed, former friends said.

“He was in his own world,” said 22-year-old Nick Pacileo.

“There were a lot of red flags,” added another former Highland Park High School classmate, who asked not to be identified.

Pacileo said he and Crimo bonded over skateboarding and were friends from eighth through 10th grade. He used words like “timid” and “quiet” to describe Crimo’s personality. He said the suspected rooftop sniper never talked about guns and would “zone out a lot.”

But when Crimo turned 18, right around the time he broke up with his girlfriend, his personality changed, Pacileo said.

“Bobby was depressed,” Pacileo said. “He also went off the deep end after he broke up with his girlfriend a few years ago.”

Crimo was obsessed with her, he said, and instead of therapy he turned to drugs.

“He definitely thought there was a border in the mind that needed to be broken through the mind,” he said. “Very third-eye type of stuff that kind of goes along with the psychedelic rap and drugs.”

Crimo also had a strained relationship with his parents, who struggled to make ends meet in the ritzy suburb, Pacileo said.

The suspect’s mother declined to discuss her son when an NBC News reporter approached her for comment.

“Get off my property,” she said outside her home before hopping into a black SUV.

Police reported Tuesday that in April 2019 they were notified after Crimo threatened to take his own life. They also received reports in September 2019 from family members saying, “Crimo was going to kill everyone.”