A video shows what a former Tennessee school bus driver was doing before he fatally crashed a bus carrying elementary school students, prosecutors say.

Johnthony Walker, then 24 years old, was driving the school bus on Nov. 21, 2016, when it flipped and crashed into a tree, killing six children and injuring others who attended Woodmore Elementary School in Chattanooga. He was accused of driving recklessly while the bus was carrying 37 students.

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During Walker's trial this week, a video from the bus was played in court that appeared to show Walker with his phone in hand as children boarded the vehicle. According to ABC News, prosecutor Crystle Carrion argued, "It all could have been avoided if Johnthony Walker had slowed down below the speed limit and stayed off his phone."

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This week, Walker pleaded not guilty to six counts of vehicular homicide and other charges – 34 in total, ABC News reported. His attorney, Amanda Dunn, has argued that the police did not investigate whether or not Walker swerved the bus to avoid hitting an oncoming white van. Dunn added that there is "no justice in convicting a man based on conjecture, and that is what the state wants to do."

In the initial reports following the incident, one parent claimed that Walker asked the kids, "Are you all ready to die?" just before the crash. Another parent, whose daughter died, said she repeatedly complained about Walker's behavior months prior to the incident. She said Walker would torment the students by doing things like "slamming on brakes on purpose, making all the kids hit their heads."

It was also revealed that his license was previously suspended for a month in 2014.

Authorities believe speed was a factor in the deadly incident, but have ruled out drugs and alcohol as explanations for the crash.