CHATTANOOGA, Tenn. -- A school bus slammed into a tree and split apart Monday, killing at least six children in Chattanooga, Tennessee, officials said.
Frantic parents arrived at the scene, screaming and crying.
A spokeswoman for the Hamilton County District Attorney's Office confirmed the death toll. But Chattanooga Police Chief Fred Fletcher would only say "multiple children" were killed, declining to specify how many died because the scene was still active.
Fletcher said the bus, which was carrying 35 students in kindergarten through fifth-grade from Woodmore Elementary School, turned over on its side and struck the tree.
Authorities received a call about the crash just before 3:30 p.m., and emergency officials worked for "many hours" to remove all the children from the bus, he said.
Fletcher said 23 victims were transported to Erlanger Hospital in Chattanooga.
Speed is a possible factor
"Certainly, speed is being investigated very, very strongly as a factor in this crash," Fletcher said, adding that the investigation was still at an early stage.
He added that driving conditions were clear and dry.
Fletcher said "a warrant has been issued to remove the informational box on the school bus and review the video."
Police said the bus driver is cooperative and talking to investigators.
Fletcher said the families of the victims were still being notified Monday night.
"Taking care of the children that are injured is our No. 1 priority. Taking care of the families who are looking for, caring for and grieving for children is our second priority," Fletcher said.
He added: "Our thoughts, our prayers and all of our efforts are with the families of these children and others impacted by this tragedy."
The National Transportation Safety Board said it would send a team to Chattanooga early Tuesday, according to a tweet from the federal agency's account.
'That's my baby'
At the scene, parents were heard crying and screaming "that's my baby" as they arrived, CNN-affiliate WTVC reported.
A woman who lives near the crash told CNN affiliate WDEF the impact was so powerful it knocked her power out.
"I just heard a big boom," the woman said.
Fletcher, in a press conference, called the crash scene "a complicated crime scene" that covered a large area.
"What has happened today is every public safety official's absolute worst nightmare, but that is nothing in comparison to the nightmare that families and friends and our community is going through with this tragic loss to children in our community," Fletcher said.
Melydia Clewell, the spokeswoman from the district attorney's office, said at least five children died on the scene, and one died at a hospital.
Tennessee Sen. Bob Corker called the crash "heartbreaking" in a tweet.
A tweet from the Chattanooga Fire Department showed the bus on its side and up against a tree as emergency officials extricated the last patient through the back door.
'Hundreds and hundreds' of donors giving blood
Blood Assurance, a blood bank in the Chattanooga area, extended its hours at three locations, as "hundreds and hundreds" of donors flocked to give blood to treat crash victims, said Mindy Quinn of Blood Assurance.
"We have had people who were refusing to leave until they give blood," Quinn said.
Quinn said they brought two bloodmobiles to one blood bank because of the huge response.
She said they were fast-tracking O-negative donors and trying to schedule donors for the days ahead and into next week.
"The blood isn't going to be needed for just tonight," she said.
Interim Hamilton County Department of Education Superintendent Kirk Kelly called the crash "one of the worst days" the school community has experienced.
"This has been a great tragedy for us. We have suffered a great loss today," Kelly said in a news conference.
He said district schools will be open Tuesday, and officials will offer support for students and staff, including guidance counselors, for as long as they are needed.
"We will do everything that we can to try to help the families involved in this tragedy," Kelly said.
The school district said it is bringing in extra counselors to offer support.
This is a developing story; refresh for updates.