Situated on a tiny block with an SRO and few permanent neighbors to complain, 16-18 Old Broadway sits between 125th and 126th Streets, surrounded by the towering Grant and Manhattanville housing projects. Cops at the 26th Precinct nicknamed the area "the Hole in the Doughnut," because it remained the lone high-crime spot even as the neighborhood around it became gentrified with chain stores and multiplexes.

The building itself was perfect for the gang's purposes. Because of neglect and its commanding view of the precinct house on 126th Street, 16-18 Old Broadway has been a bustling drug market for as long as police and its beleaguered residents can remember. Brennan believes members of the Black Top gang started out as runners for another operation years ago.

During the eighties, the building fell into disrepair at the hands of an absentee owner, Andonis Morfesis, and in 1995, the Department of Housing Preservation and Development (HPD) took it over for nonpayment of taxes. But even city oversight -- HPD has issued 627 violations to 12-14 and 529 to 16-18 -- did little to ease the gang's grip. When apartments went vacant, as they often did, the gang took them over to store drugs or weapons. "Whenever HPD would put on new locks, they'd break them and install their own," says 16-18 Old Broadway's superintendent. The gang even covered the windows with black plastic bags, which foiled police surveillance from adjoining buildings.

Under HPD's Criminal Trespass program, whereby people in a building can be arrested if they can't provide proof they have a reason to be there, hundreds of arrests were made at 16-18 Old Broadway. "But we arrested customers," Brown says.

Most of the alleged Black Top leaders lived at 16-18 Old Broadway, including brothers Angel and Jose Celpa, both 22; their mother, Gloria, who was the money courier; and their pudgy 16-year-old stepbrother, Miguel Valdez, who helped deliver the product and handle the money. (The brothers allegedly learned how to cook crack from their uncles in the Bronx.) They paid other tenants as much as $500 a month for access to their apartments, according to prosecutors.

But the gang didn't occupy the entire building, and members became ruthless when they were refused access to an apartment. "The woman in apartment 14 was spending half her time in Brooklyn, so they broke in and used her apartment as a stash house," says Brown. "She came back one time and found walkie-talkies in her baby's diapers -- she could tell that her apartment had been taken over. Two weeks later, she wanted to pick up her mail and her belongings, so she requested a police escort. We went over there and one of the locks was changed -- they actually changed one of the locks to her door. So we drilled through the lock and found one individual in there with quite a few Black Tops on him. He also had keys that belonged to the apartment underneath. And in that apartment we found 200 vials and maybe $3,000 in cash. But you know what? The next day, they were back in business."

In June 1998, Narcotics Sergeant Erin O'Reilly, an eleven-year veteran of the NYPD, was assigned to the 26th Precinct to help deal with the Black Top gang. After her role in rooting out a similar drug operation entrenched in the Castle Hotel at 106th Street and Central Park West, she earned a reputation as one of the department's more diligent investigators. "Let's put it this way," Barry says with a laugh. "I wouldn't want her investigating me."

Born in the Bronx and raised in Bergen County, O'Reilly, 41, is a cop's cop who peppers her Jersey-accented speech with lingo from Dragnet and relishes discussing the finer points of CompStat and Operation Condor. Her father is a retired New York highway patrolman who did "everything he could to keep me from becoming a police officer," O'Reilly admits a little mournfully. "You always want to see your kids do better -- I'm sure he saw his share of things that he didn't want his daughter to see.

"But I became a cop because I wanted to make a difference," she says plainly. "I know that sounds old sap, but it's really the truth." Her first assignment was to the 30th and 34th Precincts in Washington Heights at a time when "New York was the OK Corral."

By the time O'Reilly got to the 26th Precinct, veteran officers "seemed to think it was a completely futile situation," she says. "But you know what? That's what they said in the 24th Precinct, and the building next to the Castle Hotel is undergoing a $150 million renovation into condos now."

Her optimism faced its first big test in March, when Jose Celpa, arrested on a drug-sales charge, went free on a "30-30," meaning that the clock simply ran out on the allotted time to bring him to trial. "I don't know whose fault it was," O'Reilly says, but Celpa's release made it that much harder to keep her officers motivated. "It was an open-and-shut case. That really took the wind out of our sails."

Making other arrests continued to be a problem. Since the gang members had monitors that warned them when the NYPD's radio frequency was being used nearby, officers had to conduct undercover buys without being in communication. "It was extremely dangerous," O'Reilly says. "We'd have to be closer, have more eyes on the location, more officers on the roof. We'd give it a time frame as well: If they're not out in a certain time, we're rushing in. It's one of the most stressful things you can do as a supervisor. How far are you going to go to arrest a drug dealer? Are you going to sacrifice one of your officers?"

Every time police mounted a full-fledged raid, "we'd have to hide in the precinct's garage to avoid being spied by the gang's lookouts," says Brown. When officers did make it inside, lookouts would check for cops by running "verticals" on the staircase, a technique pioneered by the police themselves. "By the time we would get into the building and up to the third floor," O'Reilly explains, "the dealer would be gone. If we don't see him run into an apartment, there's nothing we can do about it."

The gang even used its own customers to run interference with officers entering the building. "Users told us that they were told by the dealers to go downstairs and intercede with police," O'Reilly says, buying gang members time to move elsewhere in the building. "One woman who ran upstairs instead of downstairs got a severe beating. And the threat of violence was always hanging over them if they talked."

Over the course of that year, O'Reilly realized that "something wasn't right in the way we were dealing with this." So in July 2000, she convened a "mini-task force" that included members of the NYPD's Gang Intelligence Unit, the 26th Precinct SNEU, Community Policing, HPD, and the district attorney's office. "I wanted to get everyone talking, throwing out ideas, pooling resources," O'Reilly says. HPD suggested that O'Reilly contact Susan Lanzatella of the Special Narcotics Prosecutor's Office. "I said, 'What can I do? What charges can I use against these individuals based on all this investigatory information that I have?' " Lanzatella suggested that conspiracy law, traditionally used against the Mafia and vast drug organizations, could be applied to a small but entrenched drug gang like Black Top.

Even if that gang wouldn't sell up, O'Reilly realized conspiracy law could give police a way to prosecute it using the bigger picture: Individuals selling $5 vials of crack thousands of times add up to a major drug organization. "What am I gonna get the businessman, the guy who runs the organization, on? I'm not going to get him on a hand-to-hand sale of narcotics," O'Reilly says. "But he is running the business. He is partaking from the profits of that business. This is where conspiracy law comes in." As opposed to arresting an individual for dealing, which requires that police observe a sale, conspiracy law allows police to use past arrest records and statements by drug buyers to prove that dealing is part of a structured business. "And that's what this was," O'Reilly maintains. "They had shifts, there was a set scale of who got paid what whether you were a lookout, whether you were an actual dealer that handed drugs to people, whether you were a runner, whether you were a cooker."

"The application of the conspiracy law in this case could be regarded as a precedent," according to Robert McCrie, chairman of the department of law, police science, and criminal justice administration at the John Jay College of Criminal Justice. It's a ground-breaking move for prosecutors, but also a potentially risky one. "Conspiracy laws have fallen out of favor, because they're hard to get convictions on -- often they're based on inferences, not evidence," McCrie says. "That might be the case when prosecuting organized crime, because you're talking about taping people meeting at a social club or observing a handshake between two parties," counters O'Reilly. "Here we've based the case on past criminal activity. How can you tell me drugs weren't sold when defendants have pleaded guilty to it in the past?"