He did not have information about the extent of injuries.
Chattanooga Police Assistant Chief Tracy Arnold said officials would not identify the students who died, or their ages, until parents were notified.
Previous reports said 12 victims were killed in the crash, but that number was not confirmed as the DA said only five victims had died by Monday night.
Some children were trapped in the bus and firefighters had to break into the vehicle to extricate the children from inside, according to WDEF-TV.
The Chattanooga Fire Department said around 5.30pm that the last patient was extricated from the bus.
It is unclear at this time what led to the crash, but police said the bus driver is cooperating with them as they try and determine what happened.
The bus was carrying students from Woodmore Elementary, ranging from kindergarten to fifth grade.
A neighbor living on Talley Road near the crash told NewsChannel 9 that the children were escaping from the roof hatch.
Fletcher said at a news conference that the bus was the only vehicle involved in the crash.
"Our hearts go out to the families, the neighborhood, the school, for all the people involved in this. We assure you we are doing everything we can," Chattanooga Police Chief Fred Fletcher said.
Fletcher said the bus was the only vehicle involved in the crash.
Fletcher said police were interviewing the bus driver to determine what happened.
Chattanooga Mayor Andy Berke told loved ones to go to the elementary school for more information.
He tweeted: "Please pray for the families of those affected by this horrible accident.'
Congressman Chuck Fleischmann shared his condolences to the families of those lost on his Facebook.
"My thoughts are with the young victims of the Woodmore Elementary School bus crash in Chattanooga, Tennessee.
"I am absolutely heartbroken over this tragedy and I will pray for the children and their families."
At the state capital in Nashville, Governor Bill Haslam called the crash "a tragic event" and offered assistance.
Blood Assurance released a statement calling for the community to donate blood.
The organisation said it is in "critical need for blood" for the 23 victims who were taken to area hospitals.
Hours for Blood Assurance have been extended so that people can donate and the organisation will also accept blood on Tuesday.
Parents received text message notifying them of the crash, according to WDEF.
"We just wanted to inform you that Hamilton County schools faced a great tragedy today."
School and county personnel are on scene of a bus accident on Tally Road in Chattanooga.
The message said that "extra counselors and support staff" would be coming in from all over the state to help students process the tragedy.
Superintendent Kirk Kelly said that schools will be open on Tuesday with guidance counselors and support at schools for students and staff, according the Times Free Press.
"This has been a great tragedy for us. We have suffered a great loss today. One of the worst days we've had in our school community," Kelly said.
- additional reporting AP
