Earlier Sunday, Donna Muller, 49, and her son Richard, 22, were found dead in their home in the 300 block of Stevens Street in Lawncrest.
They were each shot once in the head, police said. The son was shot through a pillow in his bedroom. His mother was killed in the hallway. Police say they believe the son was shot first. His mother was made to lie down with her palms on the floor under her face.
"These were execution-style murders," said Homicide Capt. James Clark.
Detectives believe the killings took place between 10:30 a.m. and 3 p.m. Sunday, Clark said.
"It appears personal - it appears to be someone they knew," he said. "The dog inside didn't attack or bark. The house was not ransacked. And there does not appear to be any forced entry."
Brian Zukowski, a friend of Richard Muller's, said Sunday night that Muller had allowed friends to stay in his basement when they needed somewhere to live.
"He was a good guy," Zukowski said. "He would take people in when they had no place to live - when they were homeless."
On Friday, Jahmeer Jett, 29, identified as the son of a police officer, was shot multiple times shortly before 6 p.m. in the block where he lived - the 6200 block of Grays Avenue in Southwest Philadelphia.
Jett died soon afterward at the Hospital of the University of Pennsylvania. Police said they did not know the motive for the slaying.
Minutes after Jett was shot, Benjamin Collier, 29, was fatally shot nearby in the 6100 block of Buist Avenue.
Collier later died at the Hospital of the University of Pennsylvania.
About 10:40 p.m. Friday, Ernest Torrence, 49, was shot to death inside his home on the 4200 block of Parkside Avenue in the Parkside section of West Philadelphia.
He was pronounced dead at the scene.
Anyone with information about a homicide can call detectives at 215-686-3334. The city offers a $20,000 reward for information leading to the arrest and conviction of suspects in homicides.