The relatives had not seen the family members for some time and had come by to check on them, only to find badly decomposed bodies.
Police said the cause of death was not obvious because the bodies were in such bad state of decomposition.
Two handguns were found at the scene, but police did not say those guns were used to commit the crime. Authorities have only confirmed that at least one person had died of a gunshot wound.
Autopsies will be performed Monday.
Detectives described the victims as an elderly woman, a man and woman in their 40s or 50s and two young women in their 20s -- a husband, wife, their two daughters and a grandmother.
There were no signs of a break-in at the home, and some neighbors said they hadn't seen the family of five since mid-May.
Investigators said murder-suicide is a possibility, and they are not looking for suspects at this time.
Residents were stunned to hear the news in the typically quiet neighborhood.
"It's just scary that two weeks would go by with no one noticing that they hadn't seen them or heard from them or anything," said neighbor Christina Cousineau.
The sheriff's department said one neighbor had called on May 14 and asked police to do a welfare check at the home. Police drove by the outside and saw nothing awry. Relatives called again on May 24 and asked for another welfare check, but asked them not to go inside because the family might be on vacation.
It wasn't until relatives made entry into the home Sunday when the dead family members were discovered.