But within seconds, as band members and politicians strolled down the street, horrified onlookers realized the noise from a nearby rooftop was a high-powered rifle firing bullets into the crowd, killing six people and injuring dozens. The attack in Highland Park, a normally safe lakeside suburb north of Chicago, prompted a massive police manhunt that forced residents to evacuate for much of the day and prompted neighboring towns to cancel their celebrations. About eight hours later, police said they arrested a 22-year-old man they described as a person of interest. Even in a country wracked by constant mass violence — in grocery stores and elementary schools and on urban street corners — the massacre in Illinois proved shocking. According to the Gun Violence Archive, a nonprofit research group, Monday’s shooting was the 15th this year in which at least four people have been fatally shot in the United States. For reasons that remained unclear to police Monday night, officials said a young man had climbed onto a rooftop with a rifle and began shooting into a sea of ​​families in lawn chairs celebrating Independence Day. “My wife looks up and screams, ‘Get up, run.’ Get up, run,” said Shawn Cotreau, 47, a Massachusetts resident visiting family in Illinois, who said he initially thought there were firecrackers nearby. Mr. Cotreau estimated that his family were sitting in chairs about 20 feet away from the gunman, who was on the roof of a store and firing. “I can’t even get the image of the man out of my head,” she said, describing a man with a large gun, wearing fatigues and a lowered hat. “He was just opening fire. And I saw the bullets hit the tree that was literally in front of us.” Police officers, already assigned to the parade route, arrived and rushed to help the injured, authorities said. The victims ranged in age from 8 to 85, said doctors who took the injured to local hospitals. The shooting stopped by the time officers arrived, police said, and the gunman was able to get away. As of late afternoon, SWAT teams were still combing the area and officials were asking residents and businesses to submit photos or videos that could provide clues. Authorities warned he was believed to be armed and told the public not to approach him. “Could it happen again?” Chief Deputy Christopher Covelli of the Lake County Sheriff’s Office asked Monday afternoon, as the search for a gunman was underway and as other Chicago suburbs scrambled to cancel parades and fireworks displays. “We don’t know what his intentions are at this point.” Robert E. Crimo III was taken into custody in nearby Lake Forest after a brief chase, police said. Survivors described a joyous family event, with high school football players and a band suddenly fighting for a living. Dr. David Baum, an obstetrician who had come to the parade to watch his 2-year-old grandson push a lawnmower along with dozens of other children, said he rushed to help after what sounded like a sonic boom and people yelling, “Bodies. down, bodies down!” Dr. David Baum, an obstetrician, rushed to help the injured victims. Statement…Mary Mathis for The New York Times Diego Rosas, who worked at a grocery store near the parade route, heard perhaps 30 gunshots and then saw people running toward the store. He let them in. Alexander Sandoval, who had arrived early with his family long before the parade started to enjoy a prime viewing spot, said he didn’t immediately understand what was happening. “When it started happening, I thought the Navy was saluting the flag,” said Mr. Sandoval, a construction contractor and lifelong Highland Park resident. “Then I grabbed my child and we ran and tried to break a window to get away from her.” Mr. Sandoval said he also tried to break down a door at a closed business, but then had to keep running. “I was banging on the door but I couldn’t break it in,” he said. “I think the shooter stopped and reloaded, and that’s when I ran around the corner and put my son and my little brother in a dumpster.” Mr. Sandoval said he saw a police officer carrying an injured boy about his own age. “It’s just emotional,” he said. The shootings prompted outpourings of sympathy from across the state and country and renewed calls from Democrats for stricter gun laws, just a week after President Biden signed the most significant gun legislation to clear Congress in decades. President Biden said he was “shocked by the senseless gun violence.” Governor JB Pritzker of Illinois, a Democrat, vowed to “end this scourge.” “There are no words for the type of monster that stalks and shoots a crowd of families with children celebrating a holiday with their community,” the governor said. “There are no words for the kind of evil that robs our neighbors of their hopes, their dreams, their future.” Those victims were Nicolas Toledo, who had recently moved to Highland Park from Mexico to spend more time with his family, according to his granddaughter, Xochil Toledo. Ms. Toledo said her family had gone out at midnight to line up the chairs so 15 of them could be together for the Fourth of July. Three in that group would be shot. Nicolas Toledo, center, was killed during the shooting in Highland Park on Monday.Credit… “We brought him here to have a better life,” Ms. Toledo said of her grandfather. “His sons wanted to take care of him and be more in his life, and then this tragedy happened.” After the shooting, 26 people were taken to Highland Park Hospital, 25 of whom suffered gunshot wounds, said Dr. Brigham R. Temple, the medical director of emergency preparedness at NorthShore University HealthSystem. At least 10 other patients were taken to nearby hospitals, he said. Their injuries ranged from minor to serious. With downtown still considered an active crime scene and residents encouraged to stay inside, many in Highland Park spoke of a mixture of shock, sadness and anger. “On the day we gathered to celebrate community and freedom, we mourn the loss, the tragic loss of life, and we grapple with the terror that was inflicted on us,” said Mayor Nancy Rotering. “I think every parent now in this community and every other community is starting to see the danger in a completely different way,” Dr. Baum said, before referring to the mass shooting in Uvalde, Texas, that left 19 children and two teachers dead at an elementary school. school. . “Uvalde is a few thousand miles away, but Uvalde happened in Highland Park in a different way.” Monday’s attack came less than eight months after the driver of an SUV plowed into a Christmas parade in Waukesha, Wis., about 80 miles northwest of Highland Park, killing six people and injuring dozens more. In Illinois, Chief Covellis said officials had recovered a rifle and that hundreds of local, state and federal law enforcement officers had searched for the gunman. The Lake County Coroner said at least five of the six people who died were adults. Officials did not immediately release their names. Highland Park Police Chief Christopher Covelli at a news conference after the shooting. Credit…Max Herman/Reuters News of the shooting rattled people across the Chicago area as officials considered whether to continue their celebrations amid the grief and manhunt. Several cities and towns canceled parades and fireworks, and even closed beaches. In Highland Park and neighboring Deerfield, residents were advised to stay indoors. Deerfield resident Michelle Bernstein said her two teenage daughters were at friends’ houses at the time of the shooting. He told them to stay there until the gunman was arrested. One of her daughters was supposed to work as a lifeguard at a local pool, she said, but the pool closed for the day. “I hope they catch the person so my kids can come home,” Ms. Bernstein said in the hours before a person of interest was taken into custody. “Right now I don’t want to go out.” And as the afternoon wore on without an arrest, signs of sudden terror remained scattered along the parade route. Abandoned carts and empty lawn chairs, some with half-consumed drinks in their cup holders, lined the sidewalk. A child’s bicycle was thrown onto the pavement. And one happy balloon, left alone on the grass, said “God bless America.” Maggie Astor, Adam Goldman, Michael Levenson, Noam Scheiber and Alan Yuhas contributed to the report.