Officers were called to a home on the 200 block of Hickory Hill Road in the far Northeast for complaints of a "strong odor" around 6 p.m., authorities said.
Inside, a 66-year-old woman was found dead in a second-floor bathroom and her 49-year-old son was found dead on the stairs, according to police.
Investigators said both bodies were "badly decomposed" and no cause of death has been confirmed. A firearm was recovered from the scene, they added.
Northeast detectives are investigating.
Two similar incidents investigated by police as murder-suicides claimed four lives in Philadelphia in a single week in May.
On May 20, PPD said 29-year-old Katie Dales was shot dead by a 27-year-old man in an Olney apartment. On May 23, authorities reported that 41-year-old Margaux Leuzzi was killed by her husband 55-year-old Joseph Messina in their shared South Philly home.
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