Bloodied students lay on stretchers, while other children walked away dazed with their parents after an elementary school bus crash that killed six Monday in Chattanooga.

Chattanooga police Chief Fred Fletcher said the crash was "every public safety professional's worst nightmare."

The bus was carrying 35 children from Woodmore Elementary, students in kindergarten through fifth grade, when it crashed about 3:30 p.m., turned on its side and wrapped around a tree.

Fletcher told reporters later that investigators were looking at speed "very, very strongly" as a factor. The bus was the only vehicle involved but the crash scene covers a significant area, he said.

Fletcher said later at news conference near where the crash occurred that the 24-year-old school bus driver, Johnthony Walker had been arrested and charged with five counts of vehicular homicide, reckless endangerment and reckless driving.

"There may be more charges added as this proceeds to the grand jury here in Hamilton County," Fletcher said.

The National Transportation Safety Board tweeted that a team would be heading to Chattanooga on Tuesday morning to investigate.

Twenty-three students were taken to hospitals, Fletcher said, but he did not have information about the extent of their injuries. Media reported it took about two hours to get the last children off the bus.

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 on Tuesday, with counselors available for students and staff.

Hamilton County District Attorney Neal Pinkston told media outlets that five people died at the scene and one died at the hospital. Melydia Clewell, the district attorney's spokeswoman, confirmed the number.

Chattanooga police Assistant Chief Tracy Arnold said officials would not identify the students who died, or their ages, until parents were notified.

"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, 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."