CHATTANOOGA, Tenn. -- What we know: Around 3:30 p.m. Monday, a school bus carrying 37 children slammed into a tree, flipped over and split apart. Families are mourning the five victims and several children remain hospitalized in intensive care. The driver has been charged with vehicular homicide and reckless driving. Authorities say the bus was exceeding the 30 mph speed limit.
What we don't know: How fast was the bus going? Why did the bus crash? What, if anything, affected the driver? What could have prevented the crash?
Investigators say they have a great deal of evidence to compile and process, including videos and interviews with the children and other witnesses. Investigators also will review videos from the two cameras on the bus, one rear-facing and one forward-facing, and an informational box on the bus to determine what caused the crash.
What we know: Of the five Woodmore Elementary School students confirmed dead, three were fourth-graders, one was a first-grader and one was a kindergartner.
One of them was D'Myunn Brown, 6, whose mother told HLN that he was an outgoing boy. He loved to talk and dance to the latest YouTube videos.
Before the crash, students celebrated Thanksgiving during lunchtime Monday, with one of the teachers taking a picture of one of the children who later did not survive. That photo was used to help identify the victim.
Injuries sustained by children on the bus included broken limbs, traumatic head and spinal injuries, and cuts and bruises, according to Erlanger Children's Hospital.
What we don't know: The identities of all of the victims.
What we know: Johnthony Walker, 24, has been charged with five counts of vehicular homicide, reckless endangerment and reckless driving. He could face more charges as the investigation unfolds.
"Mr. Walker lost control of the bus and swerved off of the roadway to the right, striking an elevated driveway and mailbox, swerved to the left and began to overturn, striking a telephone pole and a tree," according to the arrest affidavit.
After the accident, Walker called his mother, Gwenevere Cook. She said her son tried to get the children off the bus. Cook described Walker as a responsible man who took care of his 3-year-old son.
A co-worker at his other job at Amazon told CNN affiliate WSMV that Walker worked two jobs to take care of his son. "The picture that you paint of someone who'd do something like this is not the vibe I got from him," Breena Ross told the station. "He was respectable. Just a very nice person." She added that he'd often come to work tired.
What we don't know: The results of tests on a blood sample from Walker that has been sent to the state lab for processing. Were there factors, such as fatigue or distractions, that may have affected his driving?
His driving record
What we know: Walker received his commercial driving license in April, according to the NTSB.
In September, Walker was driving a school bus when he hit another vehicle, causing minor damage. The accident report cited Walker for "failure to yield right of way." The driver of the other vehicle, in an interview with the Chattanooga Times Free Press, alleged that the school bus went through a stop sign and scraped her car.
Also in 2013, Walker was cited for failure to show proof of insurance. His driver's license was suspended for three weeks. It was reinstated in late March 2014, according to state records.
What we don't know: What kind of vetting and training did Walker go through to become a school bus driver? How did the bus company investigate the September incident that Walker was involved in? Have there been prior complaints or issues reported about his driving to the company?
What we know: The bus did not have seat belts. But NTSB's chairman Christopher Hart said it takes more than seat belts to prevent injuries.
What we don't know: Could seat belts on the school bus have prevented the deaths and injuries? The NTSB will be looking into what else could have been done to lessen the extent of the damage and injuries.
Were there any mechanical issues with the bus?
The bus company
What we know: The company that operated the bus and employed Walker has come under scrutiny. Durham School Services, a company that transports more than 1 million students daily at schools across the United States, is contracted by Hamilton County.
"Our entire team at Durham School Services is devastated by the accident yesterday that tragically claimed the lives of Chattanooga students," the company said in a statement.
What we don't know: Was there proper oversight?