Inspectors say shortly after midnight Sunday, 46-year-old Patrolman Robert Penn returned to his home on Brentwood Road after working a shift.
According to officials, the 24-year veteran of the force was in plain clothes when a man approached him from behind, mumbled something to the officer, and then shot him once in the back.
The bullet went right through the officer's chest.
People heard Penn's screams for help after he was hit.
The father of two, whose wife is also a Philadelphia Police officer, was in critical condition at the Hospital of the University of Pennsylvania following the shooting. But today he was deemed well enough to be released from the hospital.
Police are continuing to search for a suspect and motive.
"[Officer Penn] changed his clothes prior to going home. So we don't know if the perpetrator knew he was a police officer at all," Deputy Commissioner Richard Ross said.
Police brought in several possible witnesses for questioning.
Philadelphia Police Commissioner Charles Ramsey tells Action News, "I don't know him personally but I don't know of any problems he's having at the moment that would lead to a shooting. But again that's all part of an investigation."
Anyone with information is asked to contact the Philadelphia Police Department.