Special needs student found dead Friday, Sept. 11, 2015, on empty school bus. (MyFoxLA)

Authorities on Saturday released the name of a 19-year-old special needs student found dead in the back of an empty school bus in a Southern California depot and said they were treating the death as "suspicious."

The student was identified as Hun Joon Lee by Lt. David Smith, of the Los Angeles County Coroner's Officer, The Los Angeles Times reported. Lee was found Friday afternoon unconscious, slumped in the aisle in the rear of the bus, MyFoxLA said.

Lee attended the Sierra Education Center in Whittier and regularly rode the bus. He typically left the school around 2:30 p.m. and arrived home at 4 p.m., said Brad White, a spokesman for the Whittier Police Department.

His mother called the school Friday when he did not return home. The school contacted the bus company. A driver went out to the bus yard across the street from the school and found the young man in the parked bus.

Police are treating the death as suspicious because "nothing that we've learned so far would lead us to believe he had any medical conditions," White said.

There were no signs of trauma on Lee's body, and no weapons were found at the scene, he said.

The bus driver was held for questioning and later released, White said, according to the Times. No arrests have been made.

Lee attended the center, which has a transition program dedicated to adult students with special needs, said Valerie Martinez, Whittier Union High School District spokeswoman.

Whittier Union High School District Superintendent Valerie Thorstenson said the Sierra staff was heartbroken, according to MyFoxLA.

"This tragedy has devastated all of us,'' she said. "We are still gathering information about what happened. The district is calling for a speedy and thorough investigation to determine how something like this could happen."

She added: "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.”

The Associated Press contributed to this report.