According to police, the room had been sealed from the inside using duct tape and a blanket. Police say they found two charcoal grills in the room containing burned charcoal and ash.
A sign hanging from the ceiling fan warned officers that they would find the man and his son in the bedroom of the home. It also warned of carbon monoxide levels.
The Attorney General's Office said autopsies determined both died from carbon monoxide poisoning and the manner of death was murder-suicide.
Derry officials said Tuesday that Preston lived with his father per a custody arrangement, but the boy was not enrolled in school.
Edmunds had also deleted his Facebook account shortly before the murder-suicide.
The family’s 6-year-old son, Connor Matthew Edmunds, died in a pool accident in March 2009. Family friends say Preston’s middle name is in honor of his older brother.
Investigators say they are looking into whether the deaths are connected.