Two parachutists were injured, one critically, in the annual Chicago Air and Water Show Saturday morning. A U.S. Navy Leap Frog jumper and a U.S. Army Golden Knight collided in the air, sending one into a nearby high-rise building and the other to the ground, CBS News reported. Neither was identified by name.

An investigation into what exactly caused the accident is ongoing. Golden Knights spokeswoman Donna Dixon told the Chicago Tribune the men were in a group of 13 executing a mass and a bomb-burst maneuver, where they held hands, broke apart and were trailed by red smoke. The Leap Frog and Golden Knight then ran into each other.

The Leap Frog fell and landed near the boathouse on North Avenue Beach, while the Golden Knight hit a high-rise on North Lake Shore Drive and East Schiller Street before tumbling to the ground.

“His legs caught the tip of the roof, and then he fell over,” witness Heather Mendenhall told the Tribune. “It was horrible.”

Both parachutists were transported to the hospital. The Leap Frog was expected to be discharged Saturday night, but the Golden Knight remained in near-critical condition. He underwent brain surgery to relieve the pressure in his head, WGN reported.

Accidents like Saturday’s are rare but not unheard-of for military parachutists. Just last month, a Golden Knight suffered unspecified injuries while rehearsing for the Milwaukee Air and Water Show.

Several Facebook users commented on the two teams’ pages with prayers for the injured parachutists on Saturday night, while others took to Twitter to express their support.

The Leap Frogs and Golden Knights canceled the rest of their performances Saturday, but the Chicago Air and Water Show was scheduled to continue Sunday. The festival, held every year on the shore of Lake Michigan, is the largest free performance of its kind, according to the website. It’s expected to draw about 1.5 million visitors.