The incident took place in Hartford County on Fairfield Drive in the town of Burlington on Sunday, around 1:25 a.m., Sunday, May 21.
State police said the truck, a 2013 Chevy Silverado, was traveling in an unknown direction when Sophie Ringquist, of Burlington, the front seat passenger, fell out from the vehicle as it was moving.
Ringquist was transported to Bristol Hospital and was later pronounced dead.
The truck was being driven by an 18-year-old Litchfield County resident, from the town of Harwinton, state police said.
"State Police were notified of the incident after the passenger had been transported to the hospital and are actively interviewing/seeking witnesses and any available video footage in an effort to reconstruct the sequence of events occurring prior to the passenger sustaining life-threatening injuries," the department said.
Initial information suggests that the passenger may have been injured as a result of a fall from the vehicle, state police said.
All of the facts and circumstances surrounding this incident remain under investigation, the department added.
Anyone with information is urged to contact Trooper Amitai Khariton or Sgt. Tim McAlpin at the Troop L barracks at 860-626-7900.
Click here to follow Daily Voice Brookfield and receive free news updates.