It was just past dinner time when the teenager was shot. Now he is face down and quiet, jeans and bloody boxer shorts cut off and cast aside. Nine girls standing in a group whisper and point.

Rumor on this block in Southeast has it that someone known as "Mustard Seed" pulled the trigger, but on the District's scale of violence, this hardly measures. The night police commander has come and gone, and the crowd, with the exception of the girls on the knoll, already is drifting. Still, there is Police Inspector Phillip O'Donnell and his officers, weaving through witnesses, trying to find out what happened and why.

It is incidents like this that drive the District's violence, and that is what O'Donnell's Rapid Deployment Unit is trying to curb. His officers, many in casual clothes, blend in and seek those who know. They pass what they get to the homicide unit or, in this case, to 7th District detectives.

"How long are you going to be in 7D?" a sergeant asks the Boston-bred O'Donnell, dressed in tan pants, black shoes and a police-blue jacket.

"Till they stop shooting so much," he says.

On paper, it is a quick-strike police squad that numbers 100 men and women, a movable force limited only by the District's boundaries and the endurance of the troops. Its mission is to reduce violent crime, no small matter in a city that has broken the homicide record set in 1989.

Police Chief Isaac Fulwood Jr. announced the new unit after nine killings in one October weekend. Three victims had been bound and shot execution-style, and Fulwood gave the unit 90 days to "bring this under control."

Reaction to the RDU -- as the month-old unit is called in the department -- was tepid. Some critics said it was a publicity stunt by a chief desperate to score a "paper" victory by staging battles. The Fraternal Order of Police said it is a retread of other units that have come and gone.

But since it began operating at the end of October, police officials said, the unit has scored significant victories. It has arrested 339 people, 199 of them for felony crimes. Four homicide cases have been closed. More than $107,000 in drugs has been seized, including $71,000 worth of cocaine. About $12,500 in cash has been seized, and, police say, the unit has confiscated 19 handguns and two shotguns. Its intelligence-gathering mission, an objective officials do not discuss publicly, also has yielded clues to networks of violence.

Few in the department argue that the concept of the force is new. In 1986, a 30-day experiment in the 5th District called Clean Sweep eventually went citywide, resulting in the arrests of hundreds of drug dealers. It also generated complaints that officers simply were dispersing dealers.

More recently, the so-called Random Roving Patrol in the 6th District logged more than 400 arrests in just over five weeks, apparently helping to reduce violent crime in troubled areas of Northeast. But the unit was short-lived, a victim of its aggressiveness. Led by Lt. Lowell K. Duckett, an outspoken officer who heads the black police caucus, the patrol received so many complaints of police brutality that the department canceled it.

Each of these new units has drawn officers with varying degrees of street experience from their own units, creating a vacuum in some cases. Privately, several police commanders question the efficiency of such operations. The Rivlin Commission, which two weeks ago issued critical reports on the police department and other city agencies, also questioned the soundness of drawing officers from their respective districts. It also suggested that such large operations, which result in hundreds of arrests, can cause more problems than they solve.

None of these criticisms is new to O'Donnell and his two senior commanders, Lt. Wayne B. Quick and Lt. Francis M. Allman. Seasoned veterans, they are not about to say the unit will solve the crime problem, once and for all. O'Donnell, a highly regarded commander with 23 years of street experience, said two weeks ago: "I can't tell you that there are 20 more murders because we're out here. I can't tell you there are 20 less murders because we're out here."

What he does say is that the unit will have an impact if allowed to perform some special functions: respond quickly to troubled areas and to all homicides, and gather intelligence that will help solve cases and eventually reduce violent crime. How profound and long-lasting the unit's influence will be, those involved in it say, depends on whether it is given a life beyond the trial period Fulwood has set.

The force is young, eager and, to a degree, inexperienced. As a result, not all has been smooth. At a recent roll call, a sergeant chastised about 40 officers for sloppy paperwork, which he said jeopardized several arrests. This, say commanders, is part of the unit's growing pains -- but is nothing time, discipline and communication cannot solve. (Being young, however, has its benefits. Three of the young officers later chased a suspect in a drug sting several blocks and into the woods off Southern Avenue SE, finally catching him in the dark.)

The force, by virtue of its size and availability, has become a ready army of the night, a source of manpower during the hours when the District experiences most of its violence. Recently, when the 1st District needed substantial backups for a raid at Potomac Gardens, the troubled housing project in Southeast, a request was made and the operation went off within hours.

Working out of a cavernous office above the department's vehicle maintenance facility in Southwest, O'Donnell and his commanders daily review the "overnights," the city's 24-hour records of violence. These statistics break down crime by hour and location, allowing the unit to track hot spots. Members have responded to some of the most publicized homicides recently, such as the slaying of a pastor, the robbery-murder of a teenager and, last week, the killing of a young mother in front of her children.

Potomac Gardens has been one of the unit's targets, and in one raid a kilo of cocaine was seized during a routine search. There is no doubt that the presence of police has an immediate impact, but it is hard to gauge just how effective this attention at Potomac Gardens will prove to be. Two days after the raid, which resulted in several arrests and the closing of a drug operation run out of one apartment, there was a double killing at the project. It is this that the unit aims to curb, even anticipate, by working the streets.

"Clearly, when they are working in an area, it has a tremendous effect," said Deputy Chief Gary L. Abrecht, who heads the 1st District. "Whether it has a residual effect, that's hard to tell."

O'Donnell said he will hit the troubled areas again and again, until there is a decrease in violent crime. He said those arrested, and those questioned at crime scenes, hold the key to unraveling the city's underground culture of violence. The theory is based, as O'Donnell says in his blunt, street-cop style, on one of the verities of crime fighting: "When you got somebody under arrest, and their backs are against the wall, they'll tell you stuff."