A Tennessee school bus driver was arrested and faces charges including vehicular homicide in a deadly crash that killed at least six people, including five children, police said in a news conference late Monday.

Chattanooga police Chief Fred Fletcher said that 24-year-old Johnthony Walker was charged with five counts of vehicular homicide. Walker was also charged with reckless driving and reckless endangerment.

Fletcher called the afternoon crash “every public safety professional’s worst nightmare.” He said earlier that investigators were looking at speed “very, very strongly” as a factor in the accident.

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Police confirmed in the overnight press conference that five children were killed in the crash. According to the Chattanooga Times Free Press, the Hamilton County District Attorney’s Office reported that an additional person had died at a hospital but it was unclear if it was another Woodmoore Elementary student.

In addition to the five deaths, six were admitted to area hospitals but weren’t critically injured and 20 others were treated and released from hospitals.

Thirty-five students from kindergarten through fifth grade were on board when the bus flipped on 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 students lay on stretchers, while others walked away dazed with their parents after the crash, local news outlets reported. Emergency personnel needed almost two hours to get all the children off the bus.

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.

"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."

The Associated Press contributed to this report.