(CNN)Local officials and the National Transportation Safety Board are investigating the deadly school bus crash in Chattanooga, Tennessee, that killed six children and wounded dozens. Here's what we know about Monday's crash:
Chattanooga bus crash: What we know -- and don't know
What we know: About 3:30 p.m. Monday, a school bus carrying 37 children slammed into a tree, flipped over and split apart. Families are mourning the six killed, 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.
The NTSB was having some problems downloading video and data from the cameras and the engine control module on the bus because of damage, according to agency chairman Christopher Hart.
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 are reviewing videos from the three cameras on the bus -- one rear-facing, one forward-facing and one facing the entrance -- and an informational box on the bus to determine what caused the crash.
Hart said investigators were looking into why the bus was not on its designated route at the time of the crash.
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.
No traces of alcohol or drugs were found in Walker's blood, lead police investigator Sgt. Austin Garrett.
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-TV 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: Were there factors, such as fatigue or distractions, that may have affected his driving?
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 in 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 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?
What we know: The company that operated the bus and employed Walker has come under scrutiny. Durham School Services, which transports more than 1 million students daily at schools across the United States, is contracted by Hamilton County.
The bus company in 2007 was given a "conditional safety rating" from the Federal Motor Carrier Safety Administration. It was later upgraded to "satisfactory" after the unspecified problems were resolved, Hart said. A website lists the most recent satisfactory rating date as July 31, 2015.
"We will be exploring what were the deficiencies," Hart said.
"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.
Durham CEO David Duke said in a YouTube statement that the company, which runs the bus service, was cooperating with the federal and local investigations.
"My responsibility now is to look for answers -- answers about why this tragedy occurred and answers for how we can make sure that this never, ever happens again," he said.
Duke said he could not comment further.
"I don't want to compromise that investigation," he said. "What I can do is promise that I'm determined, that we're determined to find out what happened. And that we will offer any support that we can to the families."
What we don't know: Was there proper oversight?