Calling the Monday afternoon crash “every public safety professional’s worst nightmare,” Chattanooga Police Chief Fred Fletcher told an overnight news conference that 24-year-old bus driver Johnthony Walker was charged with five counts of vehicular homicide. Walker was also charged with reckless driving and reckless endangerment.

Investigators were looking at speed “very, very strongly” as a factor in the crash, Fletcher said earlier. An arrest affidavit posted online by Chattanooga station WTVC says Walker was driving well above the posted 30 mph speed limit on a narrow, winding road. His bond was set at $107,500, according to the affidavit.

Thirty-five students from kindergarten through fifth grade were on board when the bus flipped onto its side and wrapped around a tree. The bus was the only vehicle involved in the crash, but Fletcher said the scene was complicated and covered a significant area. He also said a warrant had been issued to remove the bus’ black box, which contains data about the vehicle’s movement.

Bloodied Woodmore Elementary School students lay on stretchers, while others walked away dazed with their parents after the crash, local news outlets reported. More than 20 children went to hospitals for their injuries, according to Fletcher.

Television cameras showed emergency vehicles still there late into the night, and the National Transportation Safety Board tweeted that a team would be heading to Chattanooga on Tuesday morning to investigate.

Craig Harris, a parent of two children who were on the bus, told ABC’s “Good Morning America” Tuesday morning he thought the bus driver sometimes drove too fast.

Harris said his daughter and stepson were in shock and pain after the crash but were doing better Tuesday morning.

Television stations reported that people lined up to donate blood and some donors were asked to make appointments for Tuesday.

Fletcher said the families of the children who died had been notified but police would not release their names because they were juveniles.

“Our hearts go out, as well as the hearts of all these people behind me, to the families, the neighborhood, the school, for all the people involved in this, we assure you we are doing everything we can,” Fletcher said.

At the state Capitol in Nashville, Tennessee Gov. Bill Haslam called the crash “a tragic event” and offered assistance.

“We’re going to do everything we can to assist in any way,” Haslam said. “It’s a sad situation anytime there’s a school bus with children involved, which there is in this case.”