The discovery was made on Monday morning inside a home on Federal Walk in Kennett Square.
East Marlborough Township police say officers found a 55-year-old man, a 50-year-old woman and their 12-year-old child dead from apparent gunshot wounds.
It's unclear who fired the fatal gunshots.
Multiple cartridge casings were found at the scene. A semi-automatic handgun was observed on the floor near the male, according to investigators.
"This is an inconceivable tragedy for the victims' family, their loved ones, and our community. You have my deepest sympathy. We will use all our resources to determine why this heartbreaking and senseless incident happened. This is an ongoing investigation and we will update the public as soon as possible," said Chester County DA Deb Ryan.
Neighbors did not want to speak on camera, but one person believed there was tension between the couple.
The family members have not been identified.
Anyone with information is urged to contact Chester County Detective Christine Bleiler at 610-344-6866 or Sgt. Kevin Urbany at 610-444-5075.