Even the involvement of the United States attorney for the Southern District of New York, whose office prepared the search warrant application, did little to protect Mr. Brockman from the weak, contradictory, and as it turned out, utterly wrong information that led to the assault on his home.

The raid that resulted highlights not only the ways that aggressive police work can go wrong, but also the willingness -- or hesitance -- of the authorities to take responsibility for preventing such errors. At the time, the incident received no publicity and no serious attention from the police leadership.

Two days after the authorities entered Mr. Brockman's apartment, the matter of erroneous raids became inescapably public: Alberta Spruill, a 57-year-old city worker, died of a heart attack after the police burst into her apartment in Harlem on May 16 with a stun grenade, in search of a person who did not live there and who was actually in custody.

Police Commissioner Raymond W. Kelly issued a 24-page report detailing the police mistakes in the Spruill case and ordering changes in procedure. It was an effort, he said, to reduce the chances of making the same mistakes again.

The Brockman raid tests that principle of accountability against a wider field, because unlike the Spruill raid, the search of Mr. Brockman's home involved federal law enforcement as well as the police.

Mr. Kelly's spokesman, who provided a detailed account of the police involvement in the Brockman raid, said the department had started an internal affairs investigation into what went wrong.

The federal agents from the Justice Department said that the mistake was entirely the product of police information, although the police say it was the federal agents who came up with Mr. Brockman's apartment number at the last minute.

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A spokesman for prosecutors in the office of United States Attorney James B. Comey said they were not allowed to discuss what, if anything, they knew about major discrepancies in the evidence when they asked a federal magistrate for permission to knock down the door to Mr. Brockman's home.

For his part, Mr. Brockman said that while he is still rattled by the episode, he was not physically hurt during the foray, just frightened for himself and for his cats. He said the authorities should have been better prepared.

''The police -- they're all right with me,'' Mr. Brockman said. ''I suffer from seizure. A lot of times I have fell out on the street, and they picked me up. You have people who say, 'The police are dirty, this and that.' I can't find any fault with them that I know of. They got a job to do. But I don't know why they came and broke into my house. I don't see any right in that. If they have me under surveillance, they would watch me, and see who's coming in and out. Not to come in like storm troopers.''

The events leading to the search of Mr. Brockman's apartment began with a ''rash of shootings'' in the early spring, according to Inspector John Cutter of the police Intelligence Division. In response, detectives sent out an informer who they believed had proven her reliability in other cases when she bought a gun and crack cocaine under police supervision, officials said. On May 5, undercover detectives shadowed the informer as she headed into the Edenwald complex, but they were not able to track which building she entered, Inspector Cutter said.

This is where the first mistake was made, officials now say. When the informer returned, she was confused not about the apartment number, but which building she had been in.

''She comes back an hour or so later, and says, 'I was in this apartment, 1159 East 229, Apartment 5D like David, a bunch of guys sitting around wearing colors, gang colors,' '' Inspector Cutter said. ''She said, 'They had guns and it looked like a crack mill.' ''

The people in the apartment ordered the informer to leave and not come back, Inspector Cutter said, but she returned five days later, on May 10, to secretly mark the apartment door with two pieces of white tape.

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The police believed she had gone back to 1159 East 229th Street, but in fact, she had returned to the same building that she had mistaken for 1159 on her first visit, according to Michael O'Looney, the deputy police commissioner for public information.

More serious mistakes followed.

Because the informer had been inside the apartment only once, Inspector Cutter said, the police did not have enough evidence under state law to obtain a search warrant. But under federal law, he said, a single visit would suffice. So the police contacted federal prosecutors and arranged a meeting for May 13. At the meeting was an agent from the Bureau of Alcohol, Tobacco, Firearms and Explosives, which is part of the United States Justice Department.

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During that meeting, a critical misunderstanding developed: the police believed that the federal agent told them he was sending his own informer into Edenwald to verify the report from the first informant, according to Inspector Cutter.

In fact, there was no second informer, said Joseph Green, a special agent who is a spokesman for the Alcohol, Tobacco, Firearms and Explosives agency. He explained that the federal agent had an interest in the Edenwald neighborhood because he had been working on a separate investigation with other police officers. Those officers were part of the housing bureau. They were not involved in the case for which the search warrant was being prepared.

Rather than send out an informer, Agent Green said, agents called the housing bureau to double-check the address and the apartment. ''They came back and said, ''Change the apartment,' '' he said.

Instead of Apartment 5D, the housing police were telling the federal agents that it was Apartment 5F -- Mr. Brockman's home. Both apartments were wrong, since the drugs and guns were in an entirely different building, the police now say.

At the time, though, the police were convinced that their target was in 1159 East 229th Street, and they were startled to hear that the apartment number supplied by their original informant, 5D, was incorrect, Inspector Cutter said. ''That night, my detective gets a call from the ATF: 'Right building, right floor, wrong apartment.' Our detective says, 'I don't know how the informant could make that mistake.' The ATF agent said: 'My guy is good, and he says it's 5F. We're writing the warrant for 5F.' ''

The reference to ''my guy'' was a reference to a housing police officer, but the detectives had assumed the federal agent was talking about a second informer, not another police officer, according to Commissioner O'Looney.

Mr. Green, speaking for the federal agents, said, ''All the information put into the search warrant affidavit came from the N.Y.P.D.''

So less than a day before Mr. Brockman's apartment was invaded, the police believed that the federal agents had a second informer, steering them away from Apartment 5D and toward Apartment 5F. In fact, they did not. The federal agents believed they had a correct address -- given to them by a member of the housing police -- but they did not.

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Most critically, two different apartment numbers -- both in the wrong building -- had been offered as the targets.

Did the federal prosecutors who prepared the application for the search warrant know that, and if so, how did they resolve the discrepancy?

Michael J. Kulstad, a spokesman for Mr. Comey, the United States attorney for the Southern District of New York, declined to address that question directly, but said, ''Any discrepancies, if there are any, are worked out when the agent swears to an affidavit.''

When the Emergency Service Unit arrived around 7 the next morning to break down Mr. Brockman's door, the white tape used by the informant to mark the right apartment was not there, the police said. (They later found the tape on the door of an apartment in another building.)

Mr. Brockman described what happened next. ''They threw some kind of bomb in here,'' he said. ''I see these guys coming in with shields and gas masks. They told me, 'Get on the floor and on your stomach.' ''

Because of his medical problems, he said: ''It's not easy to just roll on the floor. They handcuffed me behind my back.''

As waves of officers moved through the apartment, Mr. Brockman said, they asked him about guns and drugs. When Mr. Brockman was freed, one officer remarked that he had been assigned to watch the window during the raid. ''He saw the yellow ribbon in my window, so the guys come back safe from Iraq,'' Mr. Brockman, the former marine, recalled. ''He told me, 'That's when I got a funny feeling about this.' ''