Pop-pop-pop! Along Washington's National Mall, families enjoying the Fourth of July hear explosions that should be fireworks but could be a mass shooting. They instantly scatter, a mini-panic that police quickly determine is caused by the annual crackle of patriotic celebration.

Bang-bang! In downtown Orlando, sudden noises startle a crowd and people bolt from their holiday gathering, leaving police to tweet that "there is NO evidence of a shooting in the area."

But on the same evening in Philadelphia's Center City neighborhood, similar sounds send people running — and the pops turn out to be real gunshots, which graze two police officers guarding the fireworks show in front of the Philadelphia Art Museum. Just hours earlier, a rooftop shooter in Highland Park, Ill., terrorizes a Fourth of July parade in the Chicago suburb where movie classics "Risky Business" and "Home Alone" were filmed decades ago. Seven people are killed, dozens injured and thousands left shaken, the town's main street reduced from revels to requiem in a matter of seconds.

In deadly assaults and harmless bursts of celebratory explosives, a divided nation demonstrated this holiday weekend just how anxious and jittery it has become, as the perennial flare of fireworks saluting American freedom reminded all too many people instead of the anger, violence and social isolation of the past few years.

"There is a fundamental national insecurity now, after a perfect storm of social chaos where COVID forced us to stay apart and the killing of George Floyd unleashed a movement that broke trust in the people who protect us," said Thane Rosenbaum, a lawyer and novelist who runs the Forum on Life, Culture & Society at Touro University in New York. "We're in a moral panic: 'Will anyone pick up the phone if I call for help?' Women feel more vulnerable because of the Supreme Court decision on abortion. Everyone feels more vulnerable because of soaring gas prices. People don't see a way out."

The good news about Monday's celebration of the country's 246th birthday was that large numbers of people felt safe enough to attend traditional parades, concerts and fireworks shows, many of which had been suspended or scaled back during the first two years of the coronavirus pandemic. Nearly all went off without a hitch.

On the Mall, Rachael Perrotta and Andrew Hu, both 20-year-old students at Vanderbilt University, saw no reason to leave the big crowd even after they heard about the shootings in Highland Park, and even after their parents contacted them to express concern about their safety at a large event in the middle of a big city.

Perrotta, celebrating July Fourth in Washington for the first time, said her generation has been "desensitized" to reports of violence, whether they are shootings or civil disturbances. "It's very much what we grew up with," she said. "I don't think it instills the same fear as it does for older generations, which is really sad."

But the edginess in the crowds was palpable in one city after another.

Brenda Herrera and Laleh Omaraie, 26-year-old teachers originally from Chicago, joined in the celebration on the Mall. They said gun violence is on their radar in the wake of so many mass shootings, but the problem is broader than that. Everyone seems to be on edge these days: When a bag of chips popped open on their Metro train that morning, they recalled, passengers jumped.

Viral videos of masses of people fleeing from one perceived threat or another on American streets have become a staple of social media. In many cases, as in Washington, Philadelphia and Highland Park on the Fourth, cameras capture people first looking to each other — to their friends and relatives, as well as the strangers around them — for cues before deciding whether to flee, almost as if they trust the reaction of the crowd at least as much as, if not more than, their own instincts.

In Philadelphia on Monday, the sound of gunshots prompted a rush to safety in which people slammed into others, pushing some who were fleeing to the ground and pressing others up against security fences that were meant to control the crowd. Police said they had not yet figured out who fired the shots or whether they were aimed at anyone in particular, or rather as part of a misguided holiday celebration.

The two officers who were struck were treated at a hospital and released, but the incident rattled Mayor Jim Kenney so much that he told reporters Monday night that he is looking forward to not being mayor: "It's crazy. We are the most armed country in world history and we are one of the least safe. I'm waiting for something bad to happen all the time."

In Minneapolis, an informal Fourth of July fireworks display in a public park ended at 11:30 p.m., when someone shot and wounded eight people as others fled. One man said he ducked into a hiding place at Boom Island Park's lake to escape the gunfire.

Although the country has suffered through far higher crime rates and similar periods of deep political division, "we're in uncharted territory in terms of anxiety," said Chuck Wexler, executive director of the Police Executive Research Forum and a former Boston police official. "With the George Floyd murder, war in Ukraine, the questioning of elections, people don't know who to trust. Who would think that in an iconic place like Highland Park, you would need to post snipers on rooftops on the Fourth of July? But that's what we've come to."

"Nothing feels safe anymore," he continued.

After years of violence and the social isolation brought by the pandemic, rhetoric on both shores of the national cultural and political gulf focuses on a yearning for security. On one side, it's expressed as a craving for "safe spaces" and calls for acceptance of others. On the opposite side, it's a nostalgia for a perceived golden age of social consensus.

In both cases, the political rhetoric reflects underlying jitters that lead to the kinds of street panics seen in May, when people emerging from the Barclays Center sports arena in Brooklyn stampeded toward safety after some heard pops that sounded like gunshots. In the panic, at least 10 people were injured, trampled to the pavement by others running, they thought, for their lives. In the end, there was no evidence of any gunfire.

Americans' jitters have been exacerbated in recent weeks by threats against Pride parades in Idaho, North Carolina and other states, and by last week's police shooting of a Black man in Akron, Ohio.

"We are disoriented, unable to speak the same language or recognize the same truth," social psychologist Jonathan Haidt wrote in the Atlantic in April. "We are cut off from one another and from the past."

Increasingly separated by geography, media diets and social connections, Americans have lost trust in each other and in the institutions and authorities that have traditionally brought people together, including government, police, schools, scientists and other experts, and faith and business leaders.

So when something potentially scary happens, especially in a public place that attracts a wide variety of people, the trust that once provided some assurance that all would be OK is now missing, Rosenbaum said.

"There is a longing for an America where government could be trusted to protect us," he said.

This collapse of public trust and consensus is not unique to this time: In the late 1960s and early 1970s, for example, there was similar division and racial discord, and with soaring crime rates, many people felt so insecure that they avoided certain neighborhoods or entire cities.

"But we had the will to fix things then," Rosenbaum said. "Now there's an unspoken fear that people don't feel free to talk about for fear of getting attacked on social media. If you believe now that we need more police, you can't say that publicly or you'll get destroyed on social media."

If panicky reactions on America's streets reflect a diminished trust that the police can or will keep people safe, that creates an opportunity for police to rebuild that trust, Wexler said. "Violent crime is up over the last two years, and a lot of that has to do with people being on edge and a lot of guns being out there," he said. "Small arguments become major confrontations. And when people have to figure out whether a noise is gunfire or fireworks, you see that anxiety coming out."

But when police react quickly and effectively, either by capturing the alleged shooter as they did in Highland Park on Monday night, or by immediately putting out good information about frightening moments, as Boston police did in the wake of the Boston Marathon bombings in 2013, "those are opportunities for good cops to start to regain public trust," Wexler said.

In an ever-accelerating culture characterized by the instant spread of raw and often wrong information on social media, speedy responses are the key to easing or averting panics, according to researchers who've studied trust in police and other authorities.

"As we saw in Uvalde, it's delay and lack of information that leads to mistrust and fear," Wexler said, referring to police officers' delayed response during the Texas school shooting in May.

In Washington on the Fourth, small protests popped up here and there — against the Supreme Court's overturning of the Roe v. Wade protections of abortion rights and against coronavirus vaccines — yet the larger crowds took the demonstrations in stride. Some in the crowd said they were worried about another attack like the Jan. 6, 2021, insurrection in Washington, or about random mass shootings, but others said they felt safe because of the large police presence a short distance from the protesters.

"I see a lot of officers around," said Kelly Silva, 38, as she sat amid a grove of trees near the Washington Monument, watching over a picnic blanket and a box of chicken wings. Silva, a District resident, was happy to be back at the fireworks "because everything's coming back to normal. . . . Everyone was scared two years ago, but now everybody's back. We can laugh and celebrate like before."

On this night, the fireworks were merely bombs bursting in air, the flag still there.

The Washington Post's Gaya Gupta, Caroline Pineda and Daniel Wu contributed to this report.