Warning Signs About Chattanooga School Bus Driver Weeks, Months Before Fatal Crash

— -- Weeks and even months ago, there were warning signs about the school bus driver who was charged in connection with a crash that killed six children and injured dozens of others in Tennessee on Monday.

Here's what is known so far.

Parent Complained Weeks Ago

Jasmine Mateen said three of her four daughters were on the school bus when it crashed Monday afternoon, shortly after departing from Woodmore Elementary School in Chattanooga. Two of them were released from the hospital Tuesday afternoon but the other, 6-year-old Zyaira, died at the scene.

Mateen said she had previously complained about the bus driver, identified as 24-year-old Johnthony Walker, because her children had described some behavior she found troubling.

Mateen said she made multiple phone calls to officials and wrote a letter to the school principal a few weeks ago.

"I’ve been calling the school since the first day," Mateen told ABC affiliate WTVC in Chattanooga. "Nobody’s returning my call."

The school did not immediately respond to ABC News' request to confirm whether it had received Mateen’s letter.

Previous Collision 2 Months Ago

Walker was involved in a separate bus collision two months ago, ABC News has learned.

Walker was driving a bus for Durham School Services —- as he was in Monday’s fatal crash -— in September when he sideswiped an oncoming Kia Soul after crossing into oncoming traffic while attempting to negotiate a "blind curve," according to the crash report obtained by ABC News.

There were no injuries and the passenger children were seated that day in the rear rows. The damages to both vehicles were minor, according to the crash report.

No charges were filed in the Sept. 20 incident, but Walker was cited for failing to yield right of way. It was unclear if Durham School Services took any action against Walker after that collision.

Durham School Services declined to comment to ABC News regarding the Sept. 20 incident.

License Suspended 2 Years Ago

Walker’s license was suspended for about a month in 2014 for failure to show proof of insurance, according to state commercial driver records obtained by the Associated Press.

In a news conference on Tuesday, the National Transportation Safety Board said Walker received his license to drive a school bus in April.

The NTSB, which is investigating the crash along with police, said they have not yet interviewed the driver but plan to do so.

Carina Noble, a spokeswoman for the bus company, said in a statement on Monday, "Our thoughts and prayers are with the students and families who are affected."

"Because this is an active investigation, we are unable to answer any questions at this time as we work with authorities," she added.