CHATTANOOGA, Tenn. — The school bus driver in the fatal crash here last week had strayed from his designated route, investigators have found.

Six students were killed and dozens were injured in the Nov. 21 incident. School bus driver Johnthony Walker, 24, was arrested and charged with vehicular homicide and other charges.

In a press briefing on Wednesday evening, National Transportation Safety Board (NTSB) Chairman Christopher Hart said that the crash occurred on Talley Road, which was not part of Walker’s assigned route.

“We are investigating why ... he was going a way that was not on the designated route,” Hart said. “We don’t know the answer to that yet.”

Hart described Talley as a curving road with a posted speed limit of 30 mph and a lower advisory speed for the curves. How fast the school bus was traveling at the time of the crash has not yet been confirmed, but Chattanooga Police Chief Fred Fletcher said in an earlier press briefing that "speed is being investigated very, very strongly as a factor in this crash."

Investigators have determined that the school bus departed the roadway to the right, took out a mailbox, veered to the left, struck a utility pole, and then crashed on its side into a tree, which pressed into the roof of the bus.

Hart said that investigators have been working to recover data from the bus video surveillance system and the engine control module, both of which were damaged in the crash. The engine control module could yield key information for the investigation, including the speed of the bus and any brake applications.

The bus was not equipped with seat belts, which are not required on school buses in Tennessee. Hart said that NTSB will analyze occupant protection factors, including how seat belts might have affected the outcome.

The Chattanooga Police Department reported that post-crash toxicology testing of Walker, the school bus driver, was negative for alcohol and commonly tested drugs. Still, Hart said that NTSB would send samples to its facility in Oklahoma to look at a “much longer list of drugs.”

Walker was a relatively new school bus driver, having obtained his CDL in April. According to the Chattanooga Times Free Press, multiple complaints had been lodged against him for his driving and his conduct toward students.

Hart said in the press briefing that NTSB was asking parents and others about those complaints.

“We’re going to complete our interview process before we characterize what we found,” the NTSB chairman said. “We will wait until we get the full picture.”

Investigators have found that school bus driving wasn’t Walker’s only employment.

“He did have another job with Amazon as they ramp up for their holiday shipment activity, and we are looking into whether that contributed to fatigue on his part,” Hart said.

The school bus was operated by Durham School Services under contract with the Hamilton County Department of Education (HCDE). There were 37 students from Woodmore Elementary School on board.

With students returning to school on Monday, Woodmore Principal Brenda Adamson-Cothran said in an announcement that the bus in the crash, No. 366, has been replaced with bus No. 357.

“The bus will be driven by an experienced driver,” Adamson-Cothran said. “We will have an HCDE staff member ride all of our buses for the morning and the afternoon route.”

Counselors are in place at Woodmore for students as they deal with the tragedy.

“We continue to grieve with our families who are in the midst of laying their children to rest,” Adamson-Cothran said in the announcement. “Our staff continues to support and pray for the families who still have children in the hospital and for families who have children recovering at home.”