The investigation is ongoing into the incident, which happened around 2:30 p.m. Tuesday at a home on Summerall Road in Franklin Township, Somerset County.
On Wednesday, officials identified the victim as a 2-year-old girl.
Franklin Township police responded to a call of a child in cardiac arrest, and responding officers arrived to a neighbor who is a member of a local fire department administering CPR on the unresponsive child.
Officers assisted in attempting life saving measures until multiple Robert Wood Johnson University Hospital medical units arrived on scene, but the child was subsequently pronounced dead.
Her identity has not been released.
Neighbors described the distraught parents.
"They were just screaming in pain and anguish," Treana Huntley said. "I heard the dad screaming uncontrollably, and then I heard the mom start wailing, really sobbing."
The preliminary investigation determined that the girl had been left unattended in a vehicle, though it remains unclear the exact length of time the child was in the vehicle and the circumstances surrounding the events.
It is believed the toddler was left in her car seat sometime Tuesday morning, and it wasn't until hours later that someone realized and called 911.
"It was gut-wrenching," Huntley said. "It almost made me want to break into tears, as a mother. Just hearing that pain from another mother, it was very hurtful to hear. I wouldn't wish that on anybody."
An autopsy is scheduled to be performed by the New Jersey Northern Regional Medical Examiner's Office to determine the cause and manner of death.
Anyone with additional information is urged to contact the Somerset County Prosecutor's Office Major Crimes Unit at (908) 231-7100, the Franklin Township Police Department at (732) 873-5533, or the Somerset County Crime Stoppers' Tip Line at 1-888-577-TIPS (8477).