Cardi B had just stepped offstage after performing for thousands in Central Park when a loud pop pierced the air, sounding like a gunshot and igniting fears of a shooting. Backstage, police commanders scrambled to find out what was going on, and quickly determined no shots had been fired. They rushed to the stage to tell the crowd.
“Remain calm,” Assistant Chief Kathleen O’Reilly pleaded into a microphone, saying the sound had been a fence falling over.
But it was too late. Frantic concertgoers ducked and rushed for a limited number of exits. Some people screamed “Shooter!” Barriers and tall fences were toppled. People fell and were trampled. Many fled shoeless. Some police officers even contributed to the pandemonium, telling people to duck and run.
Though no one was seriously injured, the chaos at the Global Citizen Festival on Saturday jolted law enforcement authorities, security experts and policymakers. It has forced an examination of whether the police need new ways of curbing the risk of crowd panic in an era when mass killings have heightened public fear of attacks.
By the next day, police commanders had determined that it had not been a falling barrier that had started the original stampede. It was, instead, a fight between two people near the stage. As concertgoers scattered, they stepped on empty water bottles, causing loud popping sounds.
Once the false reports of a shooting spread, controlling the crowd “was like putting toothpaste back in the tube,” Chief James R. Waters, the police counterterrorism commander, who had been on the stage, said in an interview this week.
The events in Central Park unfolded nearly a year to the day after a gunman killed 58 people at an outdoor country music concert in Las Vegas, the worst shooting in modern American history, and one in a series of mass killings at churches, concerts, newsrooms, nightclubs and schools.
“People subjectively feel like they are in greater danger than ever before,” said Steven Adelman, the vice president of the Event Safety Alliance, a nonprofit trade association.
Police officials have defended their handling of the panic in Central Park, saying the 100 officers at the concert were able to restore order within a few minutes, in part because the department has studied shooting attacks and conducted drills.
But behind the scenes, officials are grappling with what went wrong and are adopting changes that would make events like the annual music festival safer for participants during an emergency.
Those changes include marking the entrances and exits with color-coded lights, installing runway lighting to highlight emergency routes, displaying urgent messages on screens and placing specialized teams of officers in positions high enough for them to oversee the crowd, Chief O’Reilly said on Tuesday.
“Situational awareness will be what we are messaging out next year,” Chief O’Reilly said. “People have to understand where they are.”
The police commissioner, James P. O’Neill, said on Wednesday that the police could have moved faster to get a message out to calm the crowd — not just from the stage, but on social media as well. “I think our first hit on social media was about 12 minutes into it,” he said. “We can do better there.”
Mr. O’Neill said the department would “go back and take a look at what happened and see how we can prevent it in the future.”
In New York, the risk of stampedes is acute in places where crowding is common, like tourism sites and transportation hubs. Last year, Amtrak police officers set off a stampede in Pennsylvania Station when they used a Taser stun gun on a man amid delays on New Jersey Transit. Sixteen people were injured as commuters fled.
The panic in Central Park laid bare the challenges for the police and event organizers to effectively communicate with crowds during a crisis, whether it is to calm people after a false alarm, or to safely and quickly evacuate a jam-packed space. On Saturday, police commanders enlisted Chris Martin, the frontman for the band Coldplay, to get the crowd’s attention and to stress that there was no gunman.
“When people are scared, one of the first things that shuts down is their hearing,” Chief Waters said. “They get tunnel vision, their focus narrows.”
But in interviews and online posts, dozens of concertgoers laid the blame for the stampede on the festival’s organizers, security guards and the police, whom they said had contributed to the chaos with inaccurate and inadequate information about what had occurred and what the crowd should do.
“Part of the reason there was pandemonium was because the police were telling people to run and duck,” said Maria Benedek, of the East Village. “You would think in our city that people would be prepared for a situation like this.”
Several concertgoers said there were too many barriers on the Great Lawn and not enough signs. Others said the organizers seemed to have no formal evacuation procedure. “People didn’t know where to go,” said Shannon Flynn, 41.
The flood of complaints led to an apology on Sunday from Global Citizen, the event organizer. Andrew Kirk, a spokesman for Global Citizen, a charity that has a goal of ending extreme poverty by 2030, declined to comment on the group’s safety and security plans.
There was a time when loud popping sounds might not have alarmed the concertgoers who gathered on the park’s Great Lawn. But that has changed in the era of deadly mass killings, concertgoers said, and people immediately assumed there had been an attack.
Callers began flooding the city’s emergency hotline with reports of gunfire at the festival at 7:31 p.m., while the crowd of about 15,000 people was waiting for Janet Jackson to perform, the police said. Chief Waters said many of the 911 calls were people who were not at the concert relaying what they had heard from people inside.
The chaos has forced law enforcement officials to examine whether they need new ways of curbing the risk of panic in crowds in an era when mass killings have heightened public fear.
“We saw people run, fall and get trampled; people sitting down got trampled,” said Scott Hernandez, 39, a real estate agent who was standing near the back of the venue. “I heard someone say, ‘It’s a shooter,’ then we heard what sounded like gunshots, but were just other barriers being torn down. It was kind of a chain reaction. It snowballed.”
At least 37 people suffered minor injuries like sprains, cuts and bruises, the Fire Department said.
Gisselle Vazquez, 19, was standing near the front of the stage with two friends and her cousin when she heard popping noises from about 10 feet away that she mistook for fireworks.
The crowd started running. Ms. Vazquez grabbed her cousin’s hand and fled, struggling to keep up. Then someone shouted for them to duck, she said, and people began falling on top of them. She remembered thinking, “I could die right now.”
Allen Devlin, 23, a journalism student at Columbia University, saw the commotion and began recording video at 7:29 p.m. on his cellphone that shows panicked concertgoers running away from the stage in droves as police officers ran toward it. “Why are we running?” one woman said in the video.
Ms. Flynn said she heard someone shout “Shooter! Shooter! Shooter!” and the police telling people to run. So she did. “Nobody knew what was going on,” she said.
The crowd also swept up Gracie and Ellie Shanklin, teenage sisters from Millburn, N.J. They were carried along with it before running into an obstacle: the metal barriers that separated the sections and surrounded the Great Lawn.
“There were so many keeping us in the festival,” Gracie, 16, said. “It was dark, too. That didn’t help.”
The chaos lasted for nearly two full minutes on Mr. Devlin’s video before Chief O’Reilly took the microphone.
“We’ve got to get these barriers open, guys; there are people getting crushed,” she told the crowd. “Go east, or west, or backward. There’s no way for you to move to the front.”
Katherine Lee, of Astoria, Queens, and her fiancé were sitting with their shoes off on a blanket near the edge of the lawn. They tried to stand, but the rushing crowd was too close and the couple were trampled. Ms. Lee’s head hit the ground as people walked over her, she said, and her fiancé, who was also knocked down, watched helplessly in horror.
“It was the most terrifying part of the experience — feeling bodies hitting me and being walked on,” she said.
They finally escaped the park with other concertgoers after climbing over an eight-foot-tall fence. Ms. Lee sprained an ankle.
By 7:39 p.m., the police had ruled out gunshots and were ready to restart the show, Chief Waters said, but a problem with electrical power prolonged the interruption. “Our thought process was, the quicker I can get Janet on stage playing music, the quicker we get back to normalcy,” he said.
Chief Waters recalled telling the organizers that the show must go on. “Anything short of that would be, A, failure or B, surrender,” he said. “That’s not what the people came to see. That’s not what the performers came to do.”
At 8:28 p.m., the Twitter account linked to the Police Department’s public information office shared a photo of people waiting for the last performances. By then, most concertgoers had left the park and did not return or were not allowed back in.
Mr. Devlin remained at the concert with his friends until a Canadian singer, The Weeknd, closed the show at 10:30 p.m. The police presence had surged, he said, and scores of officers patrolled the entrances while paramedics removed people on stretchers.
“It never really returned to normal,” he said.
Luis Ferré-Sadurní and Jan Ransom contributed reporting. Susan C. Beachy contributed research.
A version of this article appears in print on Oct. 3, 2018, on Page A1 of the New York edition with the headline: Noise Wasn’t Gunfire, but Crowd’s Panic Was Real, and Dangerous.