WHITTIER, Calif. — A school bus driver here was arrested for the death of a student with autism who was left behind on a school bus for about seven hours on an extremely hot day in September.

On Sept. 11, student Hun Joon (Paul) Lee, 19, was supposed to be dropped off at school at about 8:30 a.m., and was expected to be dropped off near his home at around 3:30 p.m. Lee, who had a severe form of autism and was nonverbal, was found that afternoon, unresponsive, in the bus. That day was one of the hottest days of the summer, with temperatures that climbed into triple digits in some areas. School bus drivers and police were unable to revive him.

That same day, the Whittier Police Department began an investigation into Lee’s death, according to a police department press release. As a result of the investigation and in consultation with the Los Angeles County District Attorney's Office, Whittier police detectives arrested the substitute bus driver, Armando Able Ramirez, at his residence last week. He was booked at the Whittier Police Department Jail for 368PC — dependent abuse. His bail was set at $50,000 and he was arraigned last week.

Whittier police officer John Scoggins said the coroner’s investigation determined that Lee died as a result of hyperthermia, and the death was ruled accidental, Whittier Daily News reports.

The family has sued Whittier Union High School District and the bus company, Pupil Transportation Cooperative (PTC), alleging negligence in Lee’s death and in hiring Ramirez, according to the Los Angeles Times.

The lawsuit doesn’t state whether Lee was marked absent from school that day, but the plaintiffs’ attorney said that Lee’s parents were not notified of his absence, according to Whittier Daily News.

As previously reported, PTC immediately worked to upgrade its safety practices following Lee’s death, conducting an analysis of its procedures and pushing for electronic notification devices to be installed on all its buses. It was also reported that there was evidence that the driver did not conduct a standard child-check procedure, which is included in the bus operator manual, but signed off on the procedure.

Sandra Thorstenson, the school district's superintendent, issued a written statement of condolence, the Los Angeles Times reports.

"Our hearts are with our student's parents and family — we're all grieving. We're making ourselves, our counseling services and our staff available to his family and to our students and staff who were close to him."