"Apparently, there was some type of loud noise," Officer Tanya Little, a spokeswoman for the Philadelphia Police Department, said Friday morning. "From there, it just trickled."
There was no gunfire this year, Little said, and she denied a media report about a BB gun.
Police swept the area for ballistic evidence in part because of fears of gunfire, said Michael Resnick, the city's director of public safety. The sweep had "absolutely no results" in finding evidence of gunfire, he said Friday afternoon.
The city had also set up videocameras in the area, Resnick said.
"We were able to view our camera feed, and based upon that, we had negative results as well," he said. "There's no video of a shooting. You see in the video the fireworks are going off, people are hanging out, then all of a sudden there's a large crowd of people moving quickly through the area."
The ballistics search and video review, combined with what officers in that area reported, led to the conclusion that the chaos was triggered by people setting off fireworks or the echo of fireworks, Resnick said.
"I heard it. It was just a motorcycle backfire. People thought it was shooting and started stampeding," said Paul Hartigan, an officer in the area.
Thomas J. Nestel 3d, the SEPTA Transit Police chief, had another explanation when he tweeted at 11:26 p.m.: "Stampede in the 1700 block of Parkway caused by nervous people hearing fireworks reverberate off buildings. No danger!"
But imagined or not, the perceived danger had real effects.
Nervous parents carried crying children away from Logan Circle, small groups of friends huddled around streetlamps, and along the walls of Parkway buildings, people clutched each other as they looked around, asking what had just happened.
Joe Sangine, 42, said he first saw a line of about 10 people running in single file. Then the group swelled to about five wide. Then about 50 to 60 began sprinting.
"All of a sudden I saw a whole barrage of people running towards us like nothing I've ever seen before," said Austin Josiah, 20, of Orlando, Fla. He was with his friends around 20th and the Parkway.
At the Best Western Hotel on the south side of the Parkway, a young woman in a pink shirt sprinted in, tripped, and landed on her hands. "Everyone's running," she said, catching herself.
Prior to the fireworks chaos, 59 people were treated by medics, with 23 of them transported to hospitals. Afterward, 89 people were treated, Resnick said, and "the vast majority of those were for bumps, bruises, things like that - minor."
Of those, 15 were transported to hospitals. Two of them were in critical condition, according to initial medic assessments, Resnick said, though he did not know how they were evaluated at the hospitals. One person was released within 12 hours, a police spokeswoman said. The second, a woman, was in stable condition Friday afternoon.
Within moments of the crowd's rush, police began their response, directing people out of the area. A police helicopter overhead shone its searchlight on the scene, officers on bikes used them to form barricades. Police on motorcycles cleared a path.
In short, Resnick said, a large number of police officers were in the area and they all responded immediately.
"They all gathered quickly in the middle of the 1700 block," he said. "If they weren't there, I can't imagine what could have happened. It could have been worse."
"The men and women of the Police Department and Fire Department did an outstanding job maintaining safety and providing assistance to those in need," he said.
As the crowds dispersed, police, fire, and EMS personnel began to visibly relax.
There had been no repeat of last year when, toward the end of the fireworks, two teens were shot in the leg near 17th and John F. Kennedy Boulevard. In 2010, a flash mob broke out near City Hall.
Some first responders said Thursday's chaos was not unusual for the July 4 event. But there were no major incidents, they said, as they walked the Parkway, where the usual trash was dotted with shoes, apparently abandoned where people had run out of them - a black flip-flop here, a white sneaker there.
Given the size of the crowd, Resnick said Thursday, "you're going to have a little craziness."
There was only one arrest Thursday, Resnick said, for a minor, unrelated offense.
For some parents, the scariest moments were when they lost their children in the crowd. Afterward, police, using their radios to communicate across the area, were eventually able to reunite all children with their parents.
That was in some ways the bigger problem, Resnick said.
From the city's standpoint, the event was a success. Nestel, the SEPTA police chief, tweeted just before 2 a.m. that public transit had remained crime-free:
"Guessing that more than 100K people used system on July 4," he wrote. "Not one robbery, theft or assault reported anywhere on SEPTA."
Large crowds will always come with some level of danger, Resnick said, but he added that he believed the city's response went as well as it should have.
"But we can respond to mitigate the damages, which I think we were able to do. I can't say what people are going to do or not do," he said Friday. "But we try to plan for these eventualities. We man these events . . . and if something does happen we can respond quickly."
Contact Jonathan Lai at 856-779-3220, firstname.lastname@example.org, or on Twitter @elaijuh.