NEW YORK — When night falls, police officers blanket some eight odd blocks of Brownsville, Brooklyn. Squad cars with flashing lights cruise along the main avenues: Livonia to Powell to Sutter to Rockaway. And again.

On the inner streets, dozens of officers, many fresh out of the police academy, walk in pairs or linger on corners. Others, deeper within the urban grid, navigate a maze of public housing complexes, patrolling the stairwells and hallways.

This small army of officers, night after night, spends much of its energy pursuing the controversial Police Department tactic known as "Stop, Question, Frisk," and it does so at a rate unmatched anywhere else in the city. The officers stop people they think might be carrying guns; they stop and question people who merely enter the public housing project buildings without a key; they ask for identification from, and run warrant checks on, young people halted for riding bicycles on the sidewalk.

Between January 2006 and March 2010, the police made nearly 52,000 stops on these blocks and in these buildings, according to a New York Times analysis of data provided by the Police Department and two organizations, the Center for Constitutional Rights and the New York Civil Liberties Union. In each of those encounters, officers logged the names of those stopped — whether they were arrested or not — into a police database that the police say is valuable in helping solve future crimes.

These encounters amounted to nearly one stop a year for every one of the 14,000 residents of these blocks. In some instances, people were stopped because the police said they fit the description of a suspect. But the data show that fewer than 9 percent of stops were made based on "fit description." Far more — nearly 26,000 times — police listed either "furtive movement," a catch-all category that critics say can mean anything, or "other" as the only reason for the stop. Many of the stops, the data show, were driven by the police’s ability to enforce seemingly minor violations of rules governing who can come and go in the city’s public housing.

The encounters — most urgently meant to get guns off the streets — yield few arrests. Across the city, 6 percent of stops result in arrests. In these roughly eight square blocks of Brownsville, the arrest rate is less than 1 percent. The 13,200 stops the police made in this neighborhood last year resulted in arrests of 109 people. In the more than 50,000 stops since 2006, the police recovered 25 guns.

New York is among several major cities across the country that rely heavily on the stop-and-frisk tactic, but few cities, according to law enforcement experts, employ it with such intensity. In 2002, the police citywide documented 97,000 of these stops; last year, the department registered a record: 580,000.

There are, to be sure, plenty of reasons for the police to be out in force in this section of Brooklyn, and plenty of reasons for residents to want them there. Murders, shootings and drug dealing have historically made this one of the worst crime corridors in the city.

But now, in an era of lower crime rates, both in this part of Brooklyn and across the city, questions are swirling over what is emerging as a central tool in the crime fight, one intended to give officers the power to engage anyone they reasonably suspect has committed a crime or is about to.

The practice has come under intense scrutiny. Lawmakers are monitoring the situation. Civil libertarians are challenging it. The Police Department is studying it. And police officials, from Commissioner Raymond W. Kelly to local precinct commanders, are defending it.

"I don’t know what too many stops are," said Deputy Inspector Juanita Holmes, who until recently was in charge of the department’s officers specifically assigned to protect the housing projects that largely make up these eight square blocks of Brownsville. "The stops conducted by us are to address the crime, or the quality-of-life issues."

A TROUBLED NEIGHBORHOOD

In a dank stairwell inside 340 Dumont Ave., a dim light flickers and the stench of urine fills the air. It was here in 1988 in the Samuel J. Tilden Houses that Officer Anthony O. McLean took a bullet to the chest. He had been searching for a missing 10-year-old girl when he stumbled into a drug deal.

Crime here — a rectangle of housing developments and faded commercial strips — is hardly what it was in those bloody days. But the labyrinth of stairwells, hallways, courtyards, lobbies and roofs of the complexes — 65 buildings ranging from six-story apartments to 20-story towers — still presents a dangerous challenge for police.

"It’s tough," said Holmes, who took over command of these housing projects in 2008. "A lot of our kids in the area carry guns. Whether they carry them for protection, ‘because I’m trying to get to school without being victimized,’ or they carry them ‘because I’m going to rob somebody today,’ there are a lot of guns out there. And it creates a challenge."

In 2007, the year before she took command, shootings in the five developments had reached a five-year high. During her first six months on the job, her officers increased stops 23 percent from the previous six months, the data show. The next year, her officers made more than 10,000 stops.

Holmes said she studied the number of stop-and-frisks in her area closely and credited them with making the houses safer for law-abiding residents. She said over the first six months of this year violent crime was down in almost every category in the five complexes.

But that same success has not been seen outside the developments. Shootings are up 39 percent in the 73rd Precinct this year. In 2008 the precinct, which includes these blocks of public housing, led the city in murders, and it consistently has one of the city’s highest rates of violent crime.

Still, Deputy Inspector Samuel Wright, who took over command of the precinct in January 2009, says stop-and-frisks have "had a significant impact" on crime reduction.

"In 2008 there were a lot of murders in the 73rd Precinct," he said. "We were able to reduce homicides by 32 percent in 2009, and I think that was attributed to the stop, question and frisk policy. We had a reduction in robberies. We had a reduction in grand larcenies."

Law enforcement experts say that it is very hard, perhaps even impossible, to draw direct connections between the stop-and-frisk tactic and significant long-term crime reduction. Certainly, some say that the New York Police Department has so far failed to convincingly link the explosion in the numbers of stops with crime suppression.

CALLS FOR SENSITIVITY

The U.S. Supreme Court established the legal basis for stops and frisks — reasonable suspicion of a crime — in the 1968 case of Terry v. Ohio. But the officer in that case had a far different level of experience than many of the officers walking the streets of Brownsville. He had patrolled the same streets of downtown Cleveland for 30 years looking for pickpockets and shoplifters.

By comparison, the nearly 200 officers who operate in the neighborhood as part of Kelly’s "Impact Zone" program — flooding problematic crime pockets with a battery of police — are largely on their first assignment out of the academy.

The high number of stops in this part of Brooklyn can be explained in part by the fact that police can use violations of city Housing Authority rules to justify stops. For instance, the Housing Authority, which oversees public housing developments, forbids people from being in their buildings unless they live there or are visiting someone.

Inside the project buildings and out, males 15 to 34 years of age, who make up about 11 percent of the area’s population, accounted for 68 percent of the stops over the years. That amounted to about five stops a year each, though it was impossible to tell how often someone was stopped or if that person lived in the neighborhood because the data did not include the names or addresses of those stopped. Police officials say the age figures sounds right, since most crime suspects fit that description.

Many residents say they philosophically embrace the police presence. They say they know too well how the violence around them — the drugs and gangs — can swallow up young people.

Yet the day-to-day interactions with officers can seem so arbitrary that many residents say they often come away from encounters with officers feeling violated, degraded and resentful.Oddly, years ago when crime was higher, relations with the police seemed better, several residents said. The officers seemed to show a greater sense of who was law abiding and who was not, they said. Now, many residents say, the newer crop of officers are seemingly more interested in small offenses than engaging with residents.

At the local recreation center in Brownsville, Darryl Glenn, 49, stood in the gym with his son, Darryl Jr., and spoke of the need for officers to be there — but also of the equal need for them to improve their performance.

"If anything, there needs to be more police around," said the teenager, who is headed to college. But, he said, the officers could better handle some stop situations, particularly by working to get better descriptions of suspects and by communicating more effectively the reason for the stop.

HIGH NUMBER OF STOPS

The Times, for this article, interviewed 12 current or former officers who had worked in this part of Brooklyn in the past five years, and all defended the necessity of the stop-and-frisks.

But some former officers who worked the area say the stops seem less geared to bringing down crime than feeding the department’s appetite for numbers — a charge police officials steadfastly deny. Though none said they were ever given quotas to hit, all but two said that certain performance measures were implicitly expected in their monthly activity reports. Lots of stop-and-frisk reports suggested a vigilant officer.

"When I was there the floor number was 10 a month," said one officer. Like many of the officers interviewed for this article, he asked not to be identified because he was still in law enforcement and worried that being seen as critical of the New York department could hurt his future employment opportunities.

He said if you produced 10 stops — known as a UF-250 for the standardized departmental reports the stops generate — you were not likely to draw the attention of a supervisor. "And in all fairness," he said, "if you’re working in that area, 10 a month is very low. All you have to do is open your eyes."

One recent evening, the police stopped a 19-year-old man for spitting on the sidewalk, a health code violation, and entering Langston Hughes Apartments without using a key or being buzzed in, even though the doors were unlocked. "I’ve lived here for 19 years," the young man, who lived in a neighboring building, protested. "You see me coming into these buildings every day, and now you’re going to stop me."

The reaction was natural. "People don’t enjoy being stopped going to and from where they’re going," one of the officers, Robert McNamara, said later. But the officers also had another rationale, he said. They had spotted the same man near the scene of a dropped gun some days earlier, and hoped to use the stop to check for outstanding warrants. None found, they let him go without citing him, using the kind of discretion necessary in these situations, McNamara said.

Stop-and-frisk is a "valuable tool," he said, "but there needs to be some common sense when using the tool."