The parents of a 19-year-old man with autism who was found dead earlier this year after being left alone on school bus in Whittier, Calif., has filed a wrongful-death lawsuit against the bus company.

NBC4 Los Angeles reports that the family of Hun Joon "Paul" Lee contends that the company, Pupil Transportation Cooperative, had policies in place designed to prevent students from being left on buses, but failure to follow those policies led to Lee's death.

Lee was found unresponsive on the bus around 4:15 p.m. Sept. 11, a day when temperatures approached 100 degrees. The bus was parked at the company's bus depot in Whittier, according to police and fire officials.

Police say Lee rode the bus to a transition program at the Sierra Education Center near Sierra Vista High School at about 8:30 a.m. the day he died, and should have boarded it to return home by 4 p.m.

When he didn't get home on time, his mother notified the school district, which called Pupil Transportation, leading the driver to find the student unresponsive.

The lawsuit contends that Lee never got off the bus at school and the driver returned to the bus depot, locked the bus and left with Lee still inside.

Following Lee's death, bus company officials announced policy changes designed to prevent anyone from being left on a bus.