At least six children were killed and 23 others were injured when a school bus crashed into a house in Chattanooga, Tenn. Monday afternoon, officials said. 

Hamilton County District Attorney Neal Pinkston told media outlets that five people died at the scene and one died at the hospital. Pinkston’s office reported that several students were in critical condition.

Fire officials said there were approximately 35 children on the bus when the bus crashed at around 3:30 p.m. local time.

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Chattanooga Police Chief Fred Fletcher said 23 patients in all had been transported to local area hospitals, and the bus driver is cooperating with police.

The cause of the accident was not immediately clear, but Fletcher said that speed may be “one contributing factor.”

“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,” said Fletcher, who added that the crash was “every public safety professional’s worst nightmare.”

The bus was carrying students from Woodmore Elementary, who ranged from kindergarten through fifth grade. Chattanooga police Assistant Chief Tracy Arnold said officials would not identify the students who died, or their ages, until parents were notified.

A neighbor told WTVC that children were escaping from the roof hatch after the crash. The Chattanooga Times Free Press reported that two bloodied students were seen lying on stretchers in a front yard, receiving treatment from first responders. 

The paper reported that some children were able to walk away from the crash with cuts on their faces, holding their parents’ hands. Other media outlets 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.

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,” he said in a statement. “It’s a sad situation anytime there’s a school bus with children involved, which there is in this case. We will do everything we can to assist in what I think is going to be a very sad situation.”

The Associated Press contributed to this report.