While COVID-19's surge has ebbed, violence is on the rise across the United States.
There has been a rash of gun violence in what President Joe Biden called an "epidemic," including several public mass shootings, increases in incidents in major metropolitan areas and an uptick in road rage clashes.
While dramatic declines in levels of coronavirus have engendered new hope and optimism for some, the effects of the pandemic and the measures taken to combat it linger, simmering tensions brought to a boil and manifesting themselves in anger, and in some cases, violence, experts say.
Federal authorities saw that swell in violence spurred on by COVID's hardships coming -- before the pandemic even got into full swing.
An internal Department of Homeland Security memo obtained by ABC News from spring of 2020 warned that the emotional, mental and financial strain exacerbated by the new coronavirus pandemic combined with social isolation -- especially if prolonged -- may "increase the vulnerability of some citizens to mobilize to violence."
"The outbreak of Covid-19, and government's response to it, have intensified concerns that could accelerate mobilization to violence with extended periods of social distancing," the memo reads, noting such isolation is a "known risk factor" in inciting violent extremism, along with "financial stress and work disruptions, including unexpected unemployment and layoffs" also "increasing."
Even as the nation and globe was locking down, the memo, which has not been previously reported, urged agency partners to develop an "action plan" for when communities begin to return to "normal" activities, predicting "the increase in mass gatherings, combined with the lengthy social isolation and other life stressors," may create environs churned up by COVID, and ripe for violent upheaval.
When reached by ABC News regarding these early warnings, DHS declined to comment.
As a tentative reopening got underway in May, DHS Secretary Mayorkas established the Center for Prevention Programs and Partnerships, and a domestic terrorism branch in the Department's Office of Intelligence & Analysis, aimed at shoring up the Department's "whole-of-society approach" to thwarting extremism and other targeted violent acts in the U.S.
Attorney General Merrick Garland announced a "renewed commitment" and multi-pronged Justice Department effort to reduce violent crime through community engagement, targeted enforcement, and interagency collaboration.
Violent crime has "spiked since the start of the pandemic over a year ago," President Biden said in late June, announcing a range of actions and federal support towards targeting gun violence.
"And as we emerge from this pandemic with the country opening back up again, the traditional summer spike may even be more pronounced than it usually would be," Biden said.
Pandemic a 'tipping point'
It wasn't just federal officials sounding the alarm last year. Doctors -- including psychologists -- say the pressure of the pandemic may be exacerbating acts of violence and aggression.
"COVID has been a tipping point," Dr. Aimee Harris-Newon a clinical psychologist in Chicago who focuses on wellness and preventive care. "On top of too much chronic stress, the impact of all this trauma… now everything is starting to leak out."
And some experts say psychological stressors were already mounting prior to the pandemic.
"We were already in a weakened condition when the pandemic hit -- class divisions, overt racism, partisanship, a really poor social support infrastructure -- so if you think about the effect of the pandemic on an 'epidemic' of shootings -- it's like the immune system of the United States was already suppressed," Jeffrey Butts, director of the research and evaluation center at John Jay College of Criminal Justice, told ABC News.
"The social, psychological and economic distress in our country has surpassed people's ability to cope, and there hasn't been enough support," added Dr. Marni Chanoff, a psychiatrist and founder of the integrative wellness group at McLean Hospital. "There is no road map on how to navigate this time."
'COVID turned up the volume'
When Mohammed Abdelmagied heard loud bangs near his Times Square kebab and hot dog stand the last Sunday in June, he thought it was firecrackers -- someone celebrating an early Fourth of July, or maybe freedom from COVID-19.
It wasn't: it was gunfire: something he never expected in the area where he's worked for 13 years -- a heavily policed place where shootings have been relatively rare.
"I turn my face to the square, I heard everything but I didn't see nothing," Abdelmagied, 46, told ABC News.
Two shootings in two months at the Crossroads of the World have brought a flood of police to the area, in a city that until recently had become a model of safety in major metropolitan areas. These flares of gunfire aren't only in New York, nor have they remained only within city limits across the country.
Major U.S. cities have been rocked by spates of gun violence over the past few months, part of an already rising trend which did not stop during lockdown, but has become more visible as the country reopens.
"Shortly after a resumption of 'normal' life," the memo from spring of 2020 says, tensions already brewing, then exacerbated during the pandemic, may provide an opportune moment for violent extremism, and violent attacks.
Not including suicides, more than 19,400 people died by gun violence in 2020, up from roughly 15,440 in 2019, and far past the rates in years prior, according to Gun Violence Archive, a nonprofit research group.
In 2021, there have already been more than 10,000 gun violence deaths -- with nearly six months left to go.
"COVID turned up the volume," and has fomented a disintegration of social connections and norms, Butts said.
"Then we see some of these horrible shootings -- the actual magnitude of the increase is undeniable," Butts added.
It's not just gun violence on the rise: acts of aggression on airplanes have also hit new highs -- and not only more flight disruptions, but more violent ones as well.
The Federal Aviation Administration is investigating a record number of potential violations of federal law in unruly passenger cases -- identifying more than 490 cases this year so far where passengers potentially broke the law by "interfering with the duties of a crew member." That's more than double the amount of cases investigated in 2020; and more than two and a half times the amount in 2019.
Airlines have now reported more than 3,200 reports of disruptive passengers to the FAA this year; the vast majority -- more than 2,400 -- involve people who refused to wear a mask.
In a Homeland Security Threat Assessment released in October 2020, authorities also underscored concerns arising from COVID-19's impact, where "anti-government and anti-authority violent extremists could be motivated to conduct attacks in response to perceived infringement of liberties and government overreach as all levels of government seek to limit the spread of the coronavirus that has caused a worldwide pandemic."
While social media helped maintain personal connections during quarantine, it can also be quite alienating, experts say -- and present an opportunity for online radicalization.
In addition, pandemic job loss can be both heavy financial and psychological burdens.
And the unprecedented loss of life and loved ones to the virus, with more than 600,000 deaths in the U.S. alone, has taken an unspeakable toll, experts say.
Isolating factors like these can increase the risk of engaging -- or attempting to engage -- in violent extremism, according to the DHS memo.
"These risks are likely to become more widespread as public health measures are expanded-- or the timeframe for maintaining social distancing increases," the memo warned, underscoring the research-backed "need to build social links and bridges to prevent social isolation, which in turn, reduces the risk of radicalization to violence."
Social distancing has been key to stopping the virus' spread -- but after more than a year of being fearful of anyone near potentially being infected, experts point out that self-preservation may have amplified feelings of mistrust in our communities.
"Someone who's coming towards you on the sidewalk, and you'd think, you're spraying your droplets at me!" Butts said. "People were afraid. More so than before, we had to see other people as a potential deadly threat."
Americans are also still reeling from the economic and emotional blow dealt by COVID-19, despite the ebb of infection, and signs of improvement in the labor market, according to Pew polling this spring; those most vulnerable to the virus have also borne the brunt of its financial fallout.
Breaking the cycle
Tensions boiling over across the U.S. have fed what's becoming a vicious cycle difficult to break; experts worry, that residual anxiety and collective trauma may outlast the pandemic itself.
"That kind of mental and emotional wear and tear doesn't go away," Butts continued. "All the harm that results will be festering for some time. That's a huge concern."
As some Americans' anger about the state of the nation abates from where it was during the summer 2020 COVID surge -- experts urge vigilance about what that receding rage might leave in its wake.
Even as the nation prepared to celebrate the Fourth of July and some measure of freedom from COVID, federal authorities raised concerns about the possibility of domestic terror and violence, including mass shootings, as the 2021 summer season gets into full swing.
Whatever the new normal might be, Chanoff notes getting there will take time.
"The human spirit is resilient and the human capacity to heal is enormous," Chanoff said. "But without support, I think that these things will likely continue to rise."
ABC News' Josh Margolin, Aaron Katersky, Pierre Thomas and Sony Salzman contributed to this report.