The man was a resident at Woods Services, a Langhorne, Pa.-based organization that provides support and services to people with severe disabilities.
Middletown Township police say the man was left in the van on the Woods Services campus for five hours after a trip to Sesame Place.
The Bucks County coroner says the victim, Brian Nevins, originally of the New York City borough of Queens, died Sunday of hyperthermia.
A Woods Services spokeswoman says a staff person has been suspended, but the investigation continues.
Nevin's brother, who was also being cared for at the facility, has reportedly been removed by family members, CBS News Station KYW-TV in Philadelphia reports.
(Watch the report KYW-TV filed on this story below)
In November, a 17-year-old Woods resident died when he was struck by at least two cars after falling or jumping from a highway overpass. That death was ruled accidental.