The weapon was not found at the scene, according to Chief Inspector Scott Small, and the motive was unknown.
Medics rushed the victim to Temple University Hospital in critical condition, and he remained on life support there late last night, Small said.
East Detectives interviewed the victim's family yesterday.
Officially, police were treating the shooting as an aggravated assault, saying there was no indication it was a suicide attempt.
"There was no note, no weapon, no ruling at all," Small said. "All we know for certain was that he was shot in the head."
Last night, no one answered a reporter's knock on the door at the apartment. Several residents on the block said they had heard no gunshots.
One neighbor, who asked not to be identified, said he had only a cursory relationship with the victim.
"He was a nice enough guy, but no one around here really knew him," he said. "The whole situation is just strange. No one knows anything."
Small said surveillance footage from the nearby Great Wall Chinese restaurant and other businesses was being reviewed in the investigation.
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