The truck, loaded with gravel, hit the bus and flipped over. One of the special needs adults died from his injuries.
Investigators say the bus had pulled over on the narrow road to allow the truck to pass.
One of the students was killed and eight others were injured.
The people on the bus were reportedly in their 20s and from the Maryhaven Center of Hope, coming back from a day program in Riverhead. The members were headed back to their homes for the evening.
Five victims on the bus were taken by ambulance to Peconic Bay Medical Center, where the one victim died.