Numerous photos and videos showed many shunning face coverings and social distancing, two measures recommended by the Centers for Disease Control and Prevention to help stop COVID-19.
Public health officials will not know for weeks exactly how many new COVID cases are linked to the Capitol Hill riots, but they say the riots could turn out to be a super spreader event.
"I am very concerned," Dr. Angela Rasmussen, a virologist at the Georgetown Center for Global Health Science and Security, told ABC News. "Rioters were in very close proximity for long periods of time, shouting, and exposed to chemical irritants, leading to coughing. Many of them were unmasked. These are all conditions that are very conducive to [COVID-19] transmission."
This "signature pattern" seen throughout the pandemic has somehow been replayed over and over again, said Dr. John Brownstein, chief innovation officer at Boston Children's Hospital and an ABC News contributor.
"Large gatherings, with people not wearing masks, and especially if there's indoor components, are our biggest risk to spread transmission," he said, adding that potential superspreader events can have lasting effects.
Even CDC Director Dr. Robert Redfield warned on Friday that the riots likely were a "surge event" that could have public health consequences across the country.
"I do think you have to anticipate that this is another surge event. You had largely unmasked individuals in a non-distanced fashion, who were all through the Capitol," Redfield told McClatchy in an interview.
Scientists and health officials have repeatedly urged caution around mass gatherings, warning coronavirus transmission is heightened indoors, exacerbated by crowds.
"This could be the superspreader event to end all superspreader events, not to mention the fact that we are now in the worst point of this pandemic," Brownstein added.
In the first week of 2021, U.S. states and territories have reported more coronavirus cases and deaths than at any point in the pandemic, and health officials have warned that the crisis will only get worse in the coming months as the effects of increased travel and holiday gatherings are felt.
Thursday's 4,085 COVID-19-related deaths over the previous 24 hours set a new record -- the third straight record day in the U.S., according to data collected by Johns Hopkins University. With a seven-day average of 2,934 deaths by Friday, at least 368,367 Americans -- about 1 in 900 -- have died from the virus.
Many of the rioters travelled from different parts of the country and stayed in hotels. Ashli E. Babbitt, who was shot and killed inside the Capitol, was from San Diego, while three other attendees who lost their lives were from Pennsylvania, Alabama and Georgia.
"This is a huge problem, since transmission is already out of control throughout the U.S.," Rasmussen said. "It's extremely likely that multiple rioters brought COVID with them to the Capitol and spread it to others, who in turn will bring it back to their communities. Travel to and from different communities will no doubt increase opportunities for further spread. For those traveling back to communities where health care systems are already at capacity, this could be disastrous. This is exactly the opposite of what anyone should be doing right now."
Communities that may have had the pandemic relatively under control may see a resurgence when rioters return home, Brownstein explained, adding that given the discovery of the new U.K. COVID-19 variant in several states, the fallout of Wednesday's events also may accelerate the rise of this new variant.
Republican State Rep. Mike Caruso of Florida told ABC News he is concerned about people returning from the D.C. riot and spreading the virus.
"I would ask them to quarantine when they do come back and to go get tested," Caruso said, adding that "before anyone flies, they should be tested."
"It's extraordinary and negligent that rioters were not arrested and detained at the event, as this would have been an opportunity to contain new cases and prevent the spread to other communities," Rasmussen added. "It was a huge failure of law enforcement to allow the perpetrators of a violent attempted coup to continue to be a major threat to public safety as well as public health."
The mismanagement and unpreparedness of the police response led to calls by House Speaker Nancy Pelosi to fire those entrusted to protect Congress. She called the department's response "a failure of leadership." Steven Sund, head of the U.S. Capitol police, Michael Stenger, the Senate sergeant-at-arms, and Paul Irving, the House sergeant-at-arms, all resigned.
Experts also said they are worried about COVID-19 spreading to lawmakers or their staff, many of whom were barricaded in close proximity as the mob churned outside their office doors.
Rep. Susan Wild, D-Pa., told CBS News she noticed that while she and 300 to 400 people were sequestered in a secure location, many refused to wear masks.
"It's what I would call a COVID superspreader event," Wild added. "About half of the people in the room are not wearing masks. Even though they've been offered surgical masks, they've refused to wear them."
Rasmussen called the reports of unmasked legislators "unconscionable" for "anyone who purports to be in a leadership position," particularly given how newly elected Louisiana Rep. Luke Letlow died last month of COVID-19 at the age of 41.
"I fully expect new cases to result among members of Congress and their staffers," she added.
Jake LaTurner, a freshman congressman from Kansas who attended Wednesday's vote to certify the election, announced on Thursday that he had tested positive for COVID-19.
"Anyone who spent 15 or more minutes within 6 feet of Rep. LaTurner should consider themselves as a potential exposure, and should therefore quarantine for 10 days," said Andrew Noymer, a professor of public health at University of California, Irvine. "In some ways, the whole US of A is one giant superspreader event at the moment."
ABC News' Sasha Pezenik contributed to this report.