Indianapolis Public Safety Director Troy Riggs called the crash a "great tragedy."
"They were not that far from home. ... That only adds to the tragedy," Riggs said at the crash scene. He said there was no indication that the driver had a medical emergency.
Witnesses described a horrifying scene.
Duane Lloyd told WTHR that he heard a loud noise behind him as he was traveling near the intersection and saw the crash around 4:15 p.m. — about the time Chad Phelps had tweeted that the group would arrive at the church.
"I heard a skid. I looked back. I see this bus in the air and people falling out of the bus," Lloyd said. "I could have gone my whole life without seeing that."
He said people approached and tried to help.
"People were literally trying to lift the bus," Lloyd said. "You just try to do what you can do."
Sasha Sample, 28, told The Indianapolis Star some victims were lying in the road, while others were able to limp to the side.
"Everybody had boils and scrapes on them," she said. "People were trying to climb from under the bus."
Sample, a nurse, said she borrowed a belt to make a tourniquet for the bus driver's arm but wasn't able to help the man next to him, who was already dead.
"I couldn't do anything for him," Sample said. "So you triage. You help those you can."
Fire officials said 37 people were on the bus. Three teenagers were still at IU Health Methodist Hospital, Winter said Sunday, including one in critical condition. Four teenagers remained at the Riley Hospital for Children at IU Health. Many of the patients had head, arm and leg injuries, fire officials said.