The two men were taken in serious-to-critical condition to Northwestern Memorial Hospital on Saturday morning, Fire Department spokesman Juan Hernandez said.
He said witnesses told first responders that at least one of the parachutists collided with a building in the lakefront Gold Coast neighborhood, but the Fire Department had not confirmed whether that was the case.
Spectator Heather Mendenhall told the Chicago Tribune she was watching the show from a rooftop and saw one of the parachutists clip the roof next door with his feet and fall, with his parachute trailing behind him.
She said he looked unconscious as he hit the roof. A maintenance worker on the same roof called paramedics, she said.
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"If he was only one foot closer to the roof, the maintenance guy could've grabbed him," Mendenhall said. The other parachutist was found on North Avenue Beach, near the main viewing area for the show, Hernandez said.
He had no other details on the nature of the injuries or what went wrong.
One of the performers is with the U.S. Navy's Leap Frogs team, according to WBBM-TV. The station published a photo of him receiving treatment on the beach.