Speed appeared to have contributed to the 4 p.m. CST (21:00 GMT) accident, Chattanooga Police Chief Fred Fletcher said.

The accident left the bright yellow school bus wrapped around a tree, mangled and nearly severed in two. Two hours after the crash, rescue teams were still sifting through the wreckage of the bus, which was resting on its side.

Five children were found dead inside the vehicle, which normally carries 35 passengers, and a student died at a hospital, said Melydia Clewell, spokeswoman for the Hamilton County District Attorney’s office. It was not clear how many students were riding in the bus when it crashed.

The students were in kindergarten through fifth grade, she said, which would make them roughly ages 5 to 10.

There are two or three children at the hospital “who could go either way,” she said.

“Right now it appears that one contributing factor may be speed but that is part of an active, ongoing investigation,” said Fletcher.

The bus driver is being questioned and cooperating with investigators, the Chattanooga Police said on Twitter. Authorities are also investigating whether alcohol or drug use may have contributed to the crash.

Federal transportation investigators were also opening a probe into the crash, and planned to send a team to Chattanooga on Tuesday, the National Transportation Safety Board said.

School officials had not found any complaints filed against the driver, Clewell said.

Two bloodied students were lying on stretchers in a front yard nearly an hour after the crash receiving attention from first responders, while others not taken to the hospital appeared dazed with cuts on their faces, the Chattanooga Times Free Press newspaper said on its website.

Asked about the crash after a hearing in Nashville, Governor Bill Haslam said the state would offer its assistance.

“It’s a sad situation anytime there’s a school bus with children involved, which there is in this case,” he said.