Authorities searching for clues in Friday's double slaying near Lemon Hill Mansion are investigating the case as a "crime of passion," committed when someone found the victims having sex in the backseat of an SUV and shot them to death.

Police have identified Shakoor Arline, 25, and Lisa Smith, 32, as the victims found in a Toyota Sequoia on Lemon Hill Drive in Fairmount Park on Friday afternoon. Investigators are exploring whether the couple's romantic entanglements had something to do with their death.

Arline and Smith were in a relationship, but also dating other people, homicide Capt. James Clark said at a news conference Monday.

Police were working to find a man who had been dating Smith, and checking the alibi of a woman who had been dating Arline, Clark said.

"Someone knew they were in the park at that time and obviously had an issue with it," he said.

Sometime between midnight and 5 a.m. Friday, Clark said, the killer approached the SUV on foot, opened the back door, and fired nine times into the car, hitting Arline and Smith multiple times in the head.

They were found that afternoon by maintenance workers removing trash in the area.

Arline's family had been searching for him since that morning, when he failed to show up at his job at a mechanic's shop.

Eric Thomason, a family friend, said he had called Arline that afternoon over and over, to no avail. He learned that his friend was dead when Arline's mother called to tell the family that the young man found dead in Fairmount Park was her son.

"I'm still calling him," Thomason said Monday. "It hasn't hit me yet."

He said Arline loved cars and his job as a mechanic - he was drawn to "anything that went fast" - and was devoted to his family. He moved to Philadelphia from Virginia at 12. Skinny and soft-spoken, he stood out from other boys in his and Thomason's neighborhood, but "never worried about fitting in," Thomason said.

Thomason said he had never met Smith, but that Arline sometimes spoke of her, and once talked of moving with her to Arizona, her home state. The two had dated on and off for some time, Thomason said.

"He did not deserve to die like that," he said.

Investigators were pulling the couple's cellphone records Monday and running fingerprints found on the car's door, a law enforcement source said. Crime scene investigators also recovered shell casings from a .40 caliber handgun at the scene.

Anyone with any information on the case is being asked to call the homicide unit at 215-686-3334.