The parents are identified as Daniel Howarth and Felicity Westmoreland. The grandmother is identified as Donna Westmoreland.
According to Bensalem police, officers were dispatched to a report of a baby in cardiac arrest at a home on Oak Avenue in Trevose on January 7.
The 4-month-old was found unresponsive in the living room and was taken to St. Mary's Hospital in Langhorne where he was pronounced dead.
Police said an autopsy of the infant revealed the presence of fentanyl in the child's blood and the cause of death was determined to be "adverse effects of fentanyl."
According to investigators, the infant's parents, Howarth and Felicity Westmoreland, were in the home when the child died.
Officials said the home was in disarray with clothes, trash and old food. Police also discovered numerous drug baggies, syringes and a tourniquet. Those items tested positive for the presence of fentanyl and cocaine, police said.
Donna Westmoreland was previously appointed as the baby's foster mother, according to officials. A court order prohibited her from leaving the infant unsupervised with the child's parents.
Westmoreland admitted to police that she left the infant unsupervised with his parents on the day of his death.
According to police, Felicity Westmoreland and Howarth are charged with involuntary manslaughter, child endangerment, possession of a controlled substance and possession of drug paraphernalia.
Felicity and Donna Westmoreland were sent to the Bucks County Correctional Facility.
Police are searching for Howarth and have a warrant out for his arrest.
Anyone with information on the whereabouts of Howarth or information about the case is urged to call police at 215-633-3719.