They roam the city's roughest streets, where a tug at the waistband or an out-of-town license plate can stir their suspicion.

They are patrol officers who wear plainclothes and make swift arrests, snatching guns at a rate their supervisors consider prolific.

Community leaders say the mystery around the aggressive tactic creates a distrust of police, heightened in recent weeks by accusations that three undercover officers beat a Homewood teen during his arrest last month.

But law enforcement sources say the unmarked rovers -- known as "99" cars -- are among the police bureau's most valuable tools for ridding trouble spots of guns, drugs and crime.

"There would almost be no order without them," Cmdr. Catherine McNeilly said. "It would be like the Wild West."

City officers pulled a total of 1,121 guns off the street last year, police Assistant Chief Maurita Bryant said. A more telling figure when it comes to crime is the number of arrests for illegally concealing guns under the state's Uniform Firearms Act; the force made 659 gun arrests in 2009, but the police bureau does not keep statistics on how many of those were by the special undercover officers, Chief Bryant said.

At the top of last year's list for gun arrests were officers Richard Ewing, Michael Saldutte and David Sisak, who were suspended with pay last week while the city investigates a teenager's claim that they beat him. The trio made nearly 20 percent of the city's illegal firearms arrests, according to the city's police union, Officer Saldutte with 52, Officer Ewing with 47 and Officer Sisak with 32.

To make these arrests, the undercover officers have to be vigilant in their search for telltale signs that a person is concealing a weapon. They are often aggressive, and their techniques can make them prone to lawsuits and complaints of excessive force, said Phil Dacey, a retired Pittsburgh police lieutenant and former commander of the East Liberty station, where the three officers worked.

"I see very few people carrying a weapon that go easy," he said. "[The officers] are out there looking for the worst of the worst. Consequently, they do get more complaints."

Complaints such as the one made by 18-year-old Jordan Miles, a senior at the city's Creative and Performing Arts high school, erode a neighborhood's trust in police, said Rodney Lyde, pastor at Baptist Temple Church on Race Street in Homewood.

Mr. Miles, who is charged with aggravated assault and resisting arrest, said the officers did not identify themselves and attacked him without cause as he walked on Tioga Street from his mother's to his grandmother's house.

In a criminal complaint, the officers said they confronted him because he appeared to be "sneaking around" a house with heavy object in his coat that they thought was a concealed weapon. The complaint says he ran when they announced themselves and ordered him to stop walking away.

Mayor Luke Ravenstahl has asked the city's Office of Municipal Investigations to wrap up its probe into the case by the end of this month.

The Rev. Lyde said many residents wonder how undercover police decide who is suspicious enough to confront.

"Without the knowledge of how they arrive at that, people assume that it is profiling, and that it is profiling based on race," the Rev. Lyde said. "That's just the presumption that people are left to make."

That wariness, he added, can have far-reaching consequences.

"The effect is going to be that it undermines confidence," he said. "It doesn't lead to what police and citizens want, which is cooperation."

Residents want to see crime drop, he said, but they also want an explanation of the tactics police use and how they fit in to a larger plan for crime reduction.

Though they operate largely unrecognized, the undercover officers often respond directly to neighborhood complaints, which are discussed at community meetings and relayed to the officers by their commanders. Residents' reports of street-level drug dealing and other recurring trouble are the top priorities for the undercover officers, said Cmdr. McNeilly.

They stay within the boundaries of their police zone, but are not responsible for answering most 911 calls. That gives them time to focus on their main goal: to rid the zone of drugs, guns and the crimes they spawn.

"Without them, all we are are report-takers," the commander said.

They become familiar with a neighborhood's hot spots, and spend most of their time searching for crime. Suspicious behavior could be a coat with a heavily weighted pocket, a furtive movement, a broken taillight. They watch eyes and hands. They look for chances to interact with people and ask questions.

"The first thing you do is identify yourself, pull your badge out and say, 'Police, I need to talk to you,'" said narcotics and vice Lt. William Mathias. "They look at things that are out of the ordinary."

Officers chosen for the task have a range of ability and experience. Some have extra training on how to spot armed suspects. They are known by their commanders as self-starters, good at both paperwork and street work.

"Some officers are more comfortable answering 911 calls and not taking the risks these guys do," Lt. Mathias said. "Every time they approach someone who is armed, they put their lives in their own hands."

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