WHITTIER, Calif. — A 19-year-old student with special needs was found dead in an empty school bus last week, and his family believes he was left in the bus all day, Fox News reports.

The student, Hun Joon Lee, was supposed to be dropped off at the Sierra Education Center here at approximately 8:30 a.m. before leaving for home at around 2:30 p.m., last Friday, and would have been expected to be dropped off near his home at around 3:30 p.m., according to Fox News. Lee’s mother called the school when he did not come home that day, and a driver checked the bus yard and found the student, unresponsive, in the bus. School bus drivers and police tried to revive him, but were unable to do so, according to Fox 11.

Lee may have stayed on the bus for approximately nine hours because he was waiting for a cue, the student’s family members told local media. Lee had a severe form of autism and could not speak, but was able to understand instructions, NBC Los Angeles reports. The day he was found on the bus was one of the hottest days of the summer, with temperatures that climbed into triple digits in some areas.  

Lee’s family members also said that a different bus driver than usual picked Lee up that day, Fox News reports. It is not known whether the bus driver, who works for bus company Pupil Transportation Cooperative, performed a head count, according to Fox News. Police brought in the bus driver for questioning, but he was released and no arrests have been made at this time.

Whittier Union High School District Superintendent Valerie Thorstenson told Fox 11 the staff was devastated by the tragedy, and that the “district is calling for a speedy and thorough investigation to determine how something like this could happen."