The city's announcement came after President Donald Trump, who debated Democratic rival Joe Biden on Tuesday in Cleveland, revealed that he and his wife have both tested positive for Covid-19 and are in isolation. Trump was transported to Walter Reed National Military Medical Center on Friday.
"The City of Cleveland is aware of positive cases of Covid-19 following the Sept. 29 presidential debate," according to a City Hall statement. "We advise anyone who has come in contact with someone who has tested positive to self quarantine. If anyone who was in attendance has concerns or is symptomatic, they should contact their healthcare provider."
The city's announcement also came shortly after the Cleveland Clinic, which oversaw Covid-19 protocols at the debate, said it's confident that guests at Tuesday night's event will be safe from coronavirus.
"Based on what we know about the virus and the safety measures we had in place, we believe there is low risk of exposure to our guests," the Cleveland Clinic said in a statement.
The city specifically said that positive tests were traced to people involved in organizing the debate.
"In total, at this time, we are aware of 11 cases stemming from pre-debate planning and set-up, with the majority of cases occurring among out of state residents," the city said.
"At this time, though that could change, no City residents appear to have contracted the virus as a result of this event."
Despite reassurances from the Cleveland Clinic, a prominent Ohio lawmaker, who attended the debate, went into self-isolation on Friday after learning about Trump's positive test.
Ohio House Minority Leader Emilia Strong Sykes said she personally witnessed members of the president's entourage declining masks from healthcare providers — from the Cleveland Clinic — inside the hall at Case Western Reserve University.
"I am frustrated today as I worry now about my own health and the health of so many others who were present that evening like journalists, support staff, Cleveland Clinic professionals, and many others who could have potentially been exposed," Sykes said in a statement on Friday.
"This didn’t have to happen. If more would follow the guidelines, this wouldn’t continue to happen.”
Sykes' communications director Amber Epling said her boss texted senior staff that night with her concerns.
"She was just horrified, she was in the same room as them and they just waved off the physicians" offering masks, Epling told NBC News.
The Democrat from Akron, who holds advanced degrees in public health, said she doesn't know how long she'll be in quarantine. She wished Trump and the first lady a speedy recovery.
“Americans woke this morning to this news of which the ripple effects are yet to be seen. This is a very somber moment for our country in a year of somber moments," Sykes said.
"The president’s diagnosis poses a risk for not only our national security, our economic stability but also our upcoming election. I wish the president, first lady and his team a speedy recovery."
It wasn't immediately clear how many Case Western or Cleveland Clinic employees were at the site on Tuesday night, or if any of them were in close contact with Trump or White House aide Hope Hicks, who tested positive for Covid-19 on Thursday.
A spokesman for Case Western also declined comment when asked Friday if the university made any ventilation or filtration system upgrades at the Sheila and Eric Samson Pavilion ahead of Tuesday night's event.
"The university appreciates the additional efforts Cleveland Clinic is taking today for attendees who might have concerns and/or questions," Case Western said in a statement on Friday.
Some attendees, who wore masks on Tuesday night, said they were livid when mask-less spectators strolled into the hall.
Kristin Urquiza, who lost her dad to Covid-19 and was a guest of the Biden campaign on Tuesday, said she was wearing a mask — but was stunned to see no one in Trump's entourage in a facial covering.
"But when I looked over to the right-hand side, not a single person on the Trump side, including the Trump family, Melania Trump, the Trump children, were wearing a mask," Urquiza told MSNBC's "Live with Ayman Mohyeldin."
"And I remember thinking to myself, and this was just when the debate stated, 'Oh. My. Goodness. Isn't anybody going to regulate on these people wearing a mask?' And nobody did."
U.S. Rep. Tim Ryan, D-Ohio, said Trump's wife entered the hall just ahead of him but he kept a safe social distance away. But he too was amazed to see so many people without masks at the debate.
"And they walk in without masks, it was really a level of arrogance you rarely see," Ryan told MSNBC.