Services are held for Hun Joon “Paul” Lee at St. Raphael Korean Catholic Center in Norwalk on Saturday, September 19, 2015. Lee, a 19-year-old special needs student from Whittier died last week after being left on a school bus at a Whittier Union High School District parking lot in Whittier on Sept. 11, 2015. (Photo by Sarah Reingewirtz/Pasadena Star-News)

WHITTIER >> A 19-year-old special needs student from Whittier who died after being left alone on a school bus on a hot summer day last year died from hyperthermia, police said Thursday.
Prompted in part by the recent finding of Los Angeles County Department of Medical Examiner-Coroner investigators, police arrested the subcontracted substitute bus driver Wednesday on suspicion of dependent abuse in connection with the Sept. 11 death of Hun Joon “Paul” Lee, Whittier police Officer John Scoggins said.
Lee was inadvertently left alone for a matter of hours on a school bus at a Whittier Union High School school district parking lot at Mulberry Drive and Greenleaf Avenue in Whittier, as afternoon temperatures neared 100 degrees.
The coroner’s investigation, which was slowed by the need for forensic testing, ultimately determined that Lee died as a result of hyperthermia, or overheating, and the death was ruled accidental, Scoggins said.
Detectives Wednesday arrested bus driver Armando Abel Ramirez, 37, of Apple Valley. He was a substitute bus driver employed by Whittier-based Pupil Transportation Cooperative, which provided school bus serviced to the WUHSD at the time of the incident, officials said.
Wednesday’s arrest was based on, “the totality of the circumstances,” Scoggins said, though the coroner’s findings were a significant piece of the case.
Ramirez is expected to be arraigned Friday in Bellflower Superior Court. The Los Angeles County District Attorney’s Office had yet to file formal charges Thursday.
“The case is still under review, but we anticipate a filing,” Scoggins said.
Meanwhile, Ramirez was being held in lieu of $50,000 bail at the Inmate Reception Center in Los Angeles,according to county booking records.
Family members of Lee have filed a civil lawsuit against the WUHSD and PTC.
Attorney Brian Panish, of Panish Shea & Boyle LLP, which is representing the Lee family, said he was encouraged by the arrest.
“We are pleased that the Whittier Police Department has taken the first step to hold bus driver Armando Ramirez accountable for his role in the death of Paul Lee,” Panish said in a written statement. “We will continue to pursue justice for the Lee family and remain diligent in our fight to change bus safety policies and procedures in order to avoid a senseless tragedy like this from ever happening again.”
In response to Lee’s death, as well as Wednesday’s arrest, State Sen. Tony Mendoza, D-Artesia, expressed condolences Thursday to Lee’s family and friends, praised the work of investigators and announced that he had introduced legislation to try to ensure a similar tragedy is never repeated in California.
“In order to prevent these types of tragedies from happening in the future, I have introduced Senate Bill 1072, which will require every single school bus in California to be equipped with a child-safety alarm system. The bill would also require school bus drivers to receive an annual training on the system in order to get their licenses renewed,” Mendoza said in a written statement.
“It is imperative that we do everything we can to improve the safety of all children who ride a bus to school every day, not only in Whittier but throughout the state,” Mendoza said.