The City of Chicago has denied a permit for an open-water swimming event in the Chicago River set for later this year due to significant safety concerns.

The city received the permit application on June 13 for the swim, which was set for September 22. 

However, they denied it due to “significant safety concerns for participants, first responders and the general public.”

The permit application was reviewed by multiple departments, including Chicago police and fire, to identify “potential conflicts, safety issues, and the availability of resources necessary to support the event,” according to the city. 

Under municipal code, the city rejected the initial permit request, but it also offered “an alternative route that allows the event to proceed.” 

Further details haven’t been released.

Organizers said they’re still working to have the event on the planned date. Before the permit was denied, 500 swimmers had reportedly signed up to take part in the swim, according to the Sun-Times.

Proceeds from the event would support local learn-to-swim programs for at-risk youth and for amyotrophic lateral sclerosis (ALS) research at Northwestern University’s Feinberg School of Medicine.