Twenty years ago, I had the honor of observing one of the finest outfits the American police ever fielded in action: the NYPD citywide street-crime unit (SCU).

It was a relatively small group of 100 to 150 carefully selected officers with outstanding records, who prowled the city looking for those carrying guns.

And it’s a model of how the city can deal with rising gun crime without flooding the streets with additional cops or restoring stop-and-frisk.

The unit made a virtual anthropological study of street criminals. They discovered that unlike many legal gun-carriers, they didn’t generally use holsters, but instead stuck the gun in their waistbands.

Because they constantly worried about the gun falling out, they’d walk in an unusual fashion. Thus the gun-hunters looked for the subtle signs of behavior that would have escaped the notice of a regular police officer (or civilians).

If they spotted a suspected gunman, they wouldn’t jump out of their cars with guns drawn. Instead, an undercover officer would quietly work his way up to the suspect and place his hand over the gun while other officers moved in, so nobody would get shot.

The unit mastered the technique of conducting felony car stops, a tactic that has cost many police officers their lives. They never operated in a catch-as-catch-can fashion, but used choreographed tactics where several cars surrounded and halted a vehicle and officers took up designated positions around it.

Though many outstanding cops applied for a tryout with the SCU, only a certain number made it.

Members of the squad had to vote on whether to accept the newcomer. This was very serious business for them because their lives might depend upon the individuals they were working with.

The success of the SCU prompted police brass to seek to expand it.

The inspector in charge, a highly decorated street cop, argued against the proposal. He knew this kind of work wasn’t for everyone. When headquarters ignored his advice, he resigned from the department. In 1997 the unit was expanded from 138 to 438 officers.

Then disaster struck.

In February 1999, four street-crime officers were searching for a rapist in The Bronx.

When they spotted a possible suspect, Amadou Diallo, they closed in on him.

Apparently fearing a robbery, he tried to move his wallet to his pocket. The officers mistook it for a gun and fired 41 shots, 19 striking Mr. Diallo.

The officers involved in the case were young and inexperienced; they’d never worked as a team until that night.

The local district attorney indicted them for murder, but they were acquitted.

Shortly afterward, the entire street-crime unit was abolished.

In effect, the Police Department went from mistake to mistake. It wrongly expanded the unit and then wrongly abolished it.

What it should have done was restore the old SCU.

To combat the current wave of gunmen roaming the streets and pushing up our murder rate, the NYPD would benefit from bringing back stop-and-frisk to discourage criminals from carrying firearms.

But there’s a hesitancy to do so. The second problem is that while ordinary officers may occasionally be able to spot a gunman, systematic gun-hunting is a task requiring a lot of training.

A first step to reducing gun crime should be the creation of a street-crime unit modeled after the original one. This would be a group of 100 or so highly trained, motivated and carefully selected officers.

The very existence of teams of crack gun-hunters prowling the streets would be an important deterrent to illegal gun-carriers.

If the city doesn’t want to bring back the old SCU, it will send a signal that gunmen can carry their pieces without fear. If we let them carry illegal guns, they’re likely to use them.

Eventually, that means we’ll have a deluge of shootings.

Thomas A. Reppetto is the past president of the Citizens Crime Commission of New York City and a former commander of detectives in the Chicago Police Department.