PHILADELPHIA. An investigation into Saturday night's deadly shooting by an off-duty police officer could take weeks, police said yesterday, but Commissioner Charles Ramsey promised a thorough review.

Officer Frank Tepper, a 16-year veteran of the Civil Affairs Unit, was caught up in a melee involving between 20 and 40 people outside his home when he fatally shot 21-year-old William Panas, police said. Family and friends of Panas claim Tepper fired in cold blood, according to reports, while police say the officer was trying to break things up.

Authorities must determine whether Tepper, 43, was acting in self-defense.

"He has to be justified," Ramsey told 6ABC. "I mean whatever the situation is, it has to be one where you feel that your life, or the life of another, is in direct jeopardy and you need deadly force to resolve the situation."

According to police, Tepper used a personal firearm. There are conflicting reports about whether he identified himself as an officer. He has been placed on desk duty pending the investigation by police and the District Attorney's office.