The bus driver in a fatal school bus crash in Chattanooga Monday has been arrested and charged with five counts of vehicular homicide, reckless driving and reckless endangerment, Chattanooga Police Chief Fred Fletcher said late Monday night.

Fletcher identified the driver as 24-year-old Johnthony Walker.

The police chief said more charges may be added as the case proceeds to a Hamilton County grand jury.

"This is an absolute nightmare for our community, for police, for partners, but I can ensure you we are prepared and trained to make sure this is investigated thoroughly and we're making sure we have all the resources families, the schools and the communities need to respond to this tragedy and trauma of this grieving loss," Fletcher said.

Fletcher confirmed that five children died at the scene of the crash, which happened just before 3:30 p.m. Monday.

Hamilton County District Attorney Neal Pinkston said earlier Monday that there were six fatalities, which included five deaths at the scene and one at the hospital.

Fletcher said the bus, which was carrying 35 kindergarten through fifth grade students from Woodmore Elementary School, was the only vehicle involved in the crash. The bus left the road and struck a tree, he said.

Fletcher said they were still working to reunite some children with their families as of 11:30 p.m. Monday.

"We are working diligently to ensure all children who got care at the hospitals or may have been transported are reunited successfully with their families," Fletcher said.

The National Transportation Safety Board has said they are sending a team of investigators to Chattanooga Tuesday morning.

Woodmore Elementary will be open for school Tuesday, the Hamilton County Department of Education said on Facebook. Grief counselors will be at the school for students and staff.

"If parents feel it necessary to keep their students home from Woodmore Elementary, we understand, but want to once again remind you extra trained personnel will be there to counsel and aid anyone seeking assistance in dealing with this horrible tragedy," the post said.

PREVIOUS: Police in Chattanooga have confirmed that multiple children were killed in a crash Monday afternoon involving a school bus filled with elementary school children.

As of 6:30 p.m., District Attorney Neal Pinkston said there 6 fatalities, which included five deaths at the scene and one at the hospital. However, Chattanooga Police say they can not confirm that number.

"We do know that multiple children lost their lives today in this tragic incident," Chattanooga Police Chief Fred Fletcher said.

He said they were not releasing numbers of fatalities or injuries because the crash is "still an active scene."

“When we know how many children lost their lives, when we have talked to their families personally, when I have talked to their families personally, then you will have a number and not a minute before,” Fletcher said.

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While the investigation into the deadly crash is just getting underway, Fletcher said it appears one contributing factor may be the bus speed.

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

A warrant has been issued to remove the informational box on the school bus and review it.

The National Transportation Safety Board said on Twitter that they are launching an investigation of the crash, and a team of federal investigators will deploy to Chattanooga Tuesday morning.

Chief Fletcher said the bus driver is cooperating with questioning. Investigators are still on the scene of the crash gathering evidence as of 8:15 p.m.

"It is a complicated crime scene that covers a significant area," Fletcher said.

The chief said his top priority was taking care of the children who are injured and hurting. The second priority is helping the families of the deceased.

VIDEO: Multiple children killed in Chattanooga school bus crash

Officials said there were 35 students from Woodmore Elementary on board and 23 patients were transported to the hospital after what was described as a "horrible" crash that happened just before 3:30 p.m. The students are all between kindergarten and fifth grade.

The bus was the only vehicle involved in the crash. The bus turned over on its side and struck a tree, Fletcher said.

The crash comes nearly two years after a deadly school bus crash in Knox County. That deadly December 2014 crash killed two young students and a teacher's aide.

VIDEO: Officials give first update on Chattanooga school bus crash

"A bus crash involving school children is every public safety professional's worst nightmare. But it's also the thing that we all train for and you have police, fire, EMS, and our partners out here doing everything they can to rescue victims and support the families of those who've already been treated," Fletcher said.

Chattanooga blood donation centers put out a plea for donations following the deadly crash.

The community quickly answered that call.

Dozens were seen waiting in line to donate blood at one of the centers, according to a reporter from the NBC-affiliate WRCB.

The Chattanooga Fire Department said around 5:30 p.m. that the last patient was extricated from the bus.

The school bus driver is cooperating with police and their investigation, Fletcher said during a press conference.

Tennessee Highway Patrol Colonel Tracy Trott sent his condolences to the families.

"My heart goes out to the families of the children that were killed & injured in the school bus crash in Chattanooga this evening," Trott tweeted.

The fire department described it on Twitter as "very serious" and Chattanooga's mayor called it "horrible."