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 … and I have talked to their families personally, then you will have a number and not a minute more,” Chattanooga Police Chief Fred Fletcher said.

While the investigation into the deadly crash is just getting underway, Chief 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 Bureau 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.

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.

"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," Chief 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."