Philadelphians have long looked to the Police Department's Civil Affairs officers to defuse unruly confrontations like the street brawl Saturday night in Port Richmond. But that night, an off-duty Civil Affairs cop shot and killed an unarmed 21-year-old.

So how did Civil Affairs Sgt. Frank Tepper - who fired the round that killed aspiring barber William "Billy" Panas Jr. - lose his touch as a peacemaker?

That's a question Police Commissioner Charles H. Ramsey and the District Attorney's Office need to answer quickly. If the inquiry drags on into January, it will be the first major test for freshman District Attorney Seth Williams - who takes over an office that, under Lynne M. Abraham, often was slow to prosecute cops for wrongdoing.

This isn't a case where the usual one to two years should be allowed to lapse before citizens are given a full explanation why an officer resorted to deadly force.

There are just too many troubling questions about the events leading up to Panas' shooting during the 11 p.m. incident outside Tepper's own home on Elkhart Street near Edgemont.

If witness accounts are accurate, Tepper, 43, appeared drunk as he tried to break up the fight involving his relatives, including his own son.

Witnesses allege Tepper fired a gun after chasing the victim and exchanging words with him. Panas was quoted as pleading, "Come on, you're not going to shoot me." To which the officer was said to respond, "Oh, yeah?"

The official version of events is that Tepper was assaulted by several brawlers, prompting him to fire in self-defense. Clearly, that account must be tested by a thorough review of the events.

For one thing, police and prosecutors need to investigate how the incident jibes with neighbors' reports of erratic behavior at other times from the 16-year veteran.

Tepper was said to have brandished his police service weapon, fired into the air, maced neighborhood teens, and even shot at a rodent on the sidewalk.

Had those problems come to the attention of the department? And, if so, why wasn't disciplinary action - if warranted - taken earlier?

At the very least, Tepper may well have erred by wading into a brawl that involved his family. Long-standing department policy says officers should not "take action" in such disputes unless it's necessary to "prevent bodily injury or death." As described, this confrontation involved no one with a weapon - with the exception of the sergeant himself.

The Port Richmond incident is the fourth recent case in which city police officers allegedly have abused their authority, often to settle personal scores. That calls out for a broader look by Ramsey at reinforcing policies designed to prevent officers from misusing their badges.

Only a full accounting will do.