CHATTANOOGA, Tenn.  — Police arrested a Chattanooga school bus driver on charges including vehicular homicide after a bus crash killed five students.

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.

Walker is scheduled to appear in court on Nov. 29, the Times Free Press of Chattanooga reported.

Police said overnight that five children were killed in the crash. Earlier Monday, Hamilton County District Attorney Neal Pinkston told news outlets the crash killed six. The Associated Press was not immediately able to reach officials Tuesday morning to explain the discrepancy.

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.