The driver of a school bus that was filled with elementary students when it 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.

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

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 was not immediately able to reach officials early Tuesday to clarify 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.

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.

Emergency responders needed almost two hours to get all the children off the bus.

The school bus driver has been charged (Chattanooga Fire Department)

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.

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

Kirk Kelly, interim superintendent for Hamilton County schools, said classes would be held Tuesday with counselors available for students and staff.

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.