A wealthy Chicago suburb was reeling Tuesday from a devastating shooting that saw gunfire tear through a July 4 holiday crowd, as online posts and videos pointed to the troubled mind of the 21-year-old suspected gunman. Robert “Bobby” Crimo III, who grew up in Highland Park, where the shooting occurred, was arrested on Monday after six people were killed and two dozen injured during an Independence Day parade. Crimo was taken into custody after law enforcement launched a massive manhunt for the gunman who sprayed paradegoers with dozens of semi-automatic rounds from a rooftop, turning the celebration into a scene of death and trauma. “We’re all still reeling,” Highland Park Mayor Nancy Rotering told NBC’s Today show on Tuesday. “Unbelievable shock,” Rotering said. “Everybody knows somebody who was affected by this directly.” Rotering said she personally knew the suspected gunman when he was a young boy in the Cub Scouts and she was a Cub Scout leader. “How did somebody become this angry, this hateful to then take it out on innocent people who literally were just having a family day out?” Rotering asked. David Baum, a doctor whose two-year-old was in the parade, witnessed the shooting and helped treat some of the injured. “The people who were gone were blown up by that gunfire,” Baum told CNN. “The horrific scene of some of the bodies is unspeakable for the average person.” On Tuesday, police and FBI agents looking for evidence were sifting through belongings left behind by members of the crowd as they fled. Strollers, bicycles, folding chairs and other items littered the parade route through the main street of Highland Park as American flags flapped in the breeze from brick buildings. Crimo, whose father unsuccessfully ran for mayor and owns a store in Highland Park called Bob’s Pantry and Deli, was an amateur musician billing himself as “Awake the Rapper.” The younger Crimo’s online postings include violent content that alluded to guns and shootings. One YouTube video posted eight months ago featured cartoons of a gunman and people being shot. A voice-over says “I need to just do it.” It adds: “It is my destiny. Everything has led up to this. Nothing can stop me, not even myself.” Crimo, who has the word “Awake” tattooed over an eyebrow, is seen sporting an “FBI” baseball cap in numerous photos and is wearing a Trump flag as a cape in one picture. Rotering, the mayor, said the firearm used in the shooting was “legally obtained.” “This nation needs to have a conversation about these weekly events involving the murder of dozens of people with legally obtained guns,” she said. “We need to re-examine the laws.” The shooting is the latest in a wave of gun violence plaguing the United States, where approximately 40,000 deaths a year are caused by firearms, according to the Gun Violence Archive. The deeply divisive debate over gun control was reignited by two massacres in May that saw 10 Black people gunned down at an upstate New York supermarket and 21 people, mostly young children, slain at an elementary school in Texas. The Highland Park shooting cast a pall over Independence Day, when towns and cities across the country hold similar parades and people attend barbecues, sporting events and fireworks displays. In another July 4 shooting, two police officers were wounded when they came under fire during a fireworks show in Philadelphia, local officials said. In Highland Park, Emily Prazak, who marched in the parade, described the mayhem.