The wedding took place Wednesday evening at the Hilton Chicago/Northbrook, with about 150 guests in attendance, hotel officials said.
Under the state's current "tier 3" mitigation measures, meeting rooms, banquet centers, private party rooms and other venues are not allowed to host weddings at any capacity. Indoor gatherings of more than one household are also prohibited. The requirements went into effect on Nov. 20 in response to a statewide surge in COVID-19 cases.
"This event should not have happened," said Dr. Rachel Rubin, senior medical officer for the Cook County Department of Public Health, during a news conference Thursday. "It's put a lot of individuals in danger."
The hotel could face possible charges depending on the investigation, health department spokesperson Don Bolger told ABC News.
A Hilton Chicago/Northbrook spokesperson, in a statement to ABC News, said the wedding was booked before the state's latest mitigation measures went into effect, and they "sincerely regret allowing this gathering to proceed."
"The coronavirus pandemic has created unprecedented challenges for the hospitality industry and our family-owned business. However, our hope to keep the hotel afloat and employees working should not outweigh health and safety," general manager Holly Allgauer-Cir said in the statement. "We are committed to working with state and local health authorities to ensure our hotel is abiding by all safety measures and mitigation guidelines."
The hotel is also complying with contact tracing and testing protocol recommendations, as well as reviewing its bookings to "ensure any business that is inconsistent with the state's guidelines are not held at this property," Allgauer-Cir said.
Illinois Gov. J.B. Pritzker said during his daily press briefing Thursday he was "deeply concerned" about the health of the attendees and their communities, and urged guests to quarantine and get tested. According to Pritzker, there was a lack of social distancing at the wedding as COVID-19 runs "rampant" throughout Illinois.
On the day of the event, Illinois public health officials reported a record 238 deaths and 9,757 new confirmed and probable cases of COVID-19. On Thursday, there were 10,959 such cases and an additional 192 deaths.
"It's very irresponsible, and it's also irresponsible of the hotel to host an event like that," the governor said. "They too have breached the rules and they should be held responsible."
Following news of the wedding, the Illinois Hotel and Lodging Association said it reached out to its members "to once again stress the importance of abiding by all health and safety guidelines."
"The large gathering that was allowed to take place at the Hilton Chicago/Northbrook is unacceptable and does not reflect the careful efforts the hotel industry as a whole has taken since the onset of the pandemic to protect guests, employees and our communities," President and CEO Michael Jacobson said in a statement.
ABC News' Rachel Katz contributed to this report.