(AP) -- The driver of an elementary school bus that crashed in Chattanooga, killing at least five children, has been arrested and faces charges including vehicular homicide.

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.

Fletcher said earlier that investigators were looking at speed "very, very strongly" as a factor in the crash.

Police said overnight that five children were killed in the crash. Earlier in the day, Hamilton County District Attorney Neal Pinkston told news outlets the crash killed six. The Associated Press wasn't immediately able to reach officials early Tuesday to clarify the discrepancy.