BRONX, NY — Federal officials joined a widening city investigation Friday into the killing of an unarmed West African immigrant by four police officers who fired 41 shots in a ferocious barrage that remained officially unexplained even as controversy over the case spread from City Hall to the streets of the Bronx.

The United States Attorney in Manhattan, Mary Jo White, said her office would work closely with the Bronx District Attorney, Robert T. Johnson, in the inquiry into the shooting early Thursday of 22-year-old Amadou Diallo by officers who confronted him at the vestibule of his Bronx building.

Police fired 41 rounds at Diallo, hitting him 19 times.

While Ms. White offered no elaboration, legal experts said the early Federal intervention was highly unusual, perhaps intended as public reassurance in an explosive case already drawing comparisons to those of Abner Louima, the Haitian immigrant tortured in a Brooklyn station house in 1997, and Anthony Baez, who died in the Bronx after a struggle with Officer Francis X. Livoti in 1994.

Investigators were keeping an official silence, and the officers themselves have not yet been questioned. There were few facts to go on in a case that seemed to crystallize the apprehensions of New York's minority groups over the aggressive police policies of Mayor Rudolph W. Giuliani's administration. The victim was black and the four officers involved are white.

But an unofficial account was provided in an interview by Stephen Worth, a lawyer who was hired through the auspices of the Patrolmen's Benevolent Association to represent the four officers, all members of the Bronx street crimes unit, who were searching for a rapist when the fatal encounter occurred.

Diallo was carrying only a wallet and a beeper.

Mr. Worth quoted the officers as saying they were driving along Wheeler Avenue in an unmarked car when they first saw Mr. Diallo in the vestibule of his building.

He was acting suspiciously, they said, and they thought he resembled the sketch of a rapist they were seeking.

By all accounts, he had never been in trouble with the law.

The lawyer did not elaborate on the suspicious behavior, but another person familiar with the officers' account — all four gave roughly the same story — said Mr. Diallo appeared to be fumbling with something as he entered the vestibule."

He is acting strange, he fits the rapist's description in a generic way," Mr. Worth said. The plainclothes officers got out of their car, he said, approached Mr. Diallo and identified themselves."

Despite repeated identifications and orders to this individual to do certain things, he failed to comply," Mr. Worth said. He declined to say what the orders were.

In addition to his refusal to comply, Mr. Diallo exhibited "aggressive behavior," which Mr. Worth refused to define. "There may have been a language problem in terms of his understanding their directions," Mr. Worth acknowledged. Mr. Diallo's roommates said he spoke English well, but slowly and with a stutter.

In any case, the officers drew their guns — each had a 16-shot 9-millimeter semiautomatic pistol — and Mr. Diallo may have made some movement or gesture they took to be threatening, Mr. Worth said.

"... this kind of fire power is to neutralize what they perceive as a threat ..."

The other person familiar with the officers' account quoted them as saying Mr. Diallo, who was carrying only a wallet and a beeper, moved his hands and arms in a way that made the officers think he was going for a gun.

Mr. Worth said the officers then opened fire because they thought he had a gun. "The reason they are shooting him is they think he has the gun," he said.

"... it may seem to a layman to be excessive ..."

Asked about the large number of shots fired, Mr. Worth said: "The reason they are given this kind of fire power is to neutralize what they perceive as a threat. While it may seem to a layman to be excessive, it was the number required before this man stopped. All 41 shots did not hit this individual."

An autopsy yesterday found that Mr. Diallo suffered 19 gunshot wounds: 1 to the front of the chest, 5 to the left side, 1 to the left back, 1 to the right arm and 11 in the legs. Internal injuries — ruptures of the aorta, spinal cord, lungs, liver, spleen, kidney and intestines — caused his death, said Ellen Borakove, a spokeswoman for the Medical Examiner's office.

"This was not a police murder, this was a police slaughter."

A day after the shooting, as the victim's body was taken to a Harlem funeral home to be washed and wrapped to be sent home to Guinea, West Africa, for burial, there were waves of protest

and expressions of outrage by relatives and friends of Mr. Diallo, civil rights advocates and human rights organizations.

"We want justice," Kyle B. Watters, a lawyer for the victim's family, said outside 1157 Wheeler Avenue, in the Soundview section, where Mr. Diallo was slain. "Ultimately, we want to have these officers prosecuted and convicted and spend the rest of their lives in jail."

Standing beside him, the victim's uncle, Mamadou Diallo, wept and said, "I'm too sad to speak." But the Rev. Al Sharpton, called the killing "the worst form of police brutality we have ever seen or heard of."

"This was not a police murder, this was a police slaughter," Mr. Sharpton said. He called for a Federal inquiry and spoke of plans for a protest rally tomorrow outside the victim's building.

Amnesty International U.S.A., in a statement, said the slaying "raises deeply troubling questions about the use of excessive force and police brutality," and called for an independent commission to investigate. The Center for Constitutional Rights urged the appointment of a special prosecutor.

Leaders of an organization called 100 Blacks in Law Enforcement Who Care called for a Federal investigation, and contended that the street crimes unit, to which the involved officers belong, was a danger to New Yorkers.

"That is the unit that's been given carte blanche to do as it will to the people of the City of New York, especially the African-American community," Eric Adams, a city police lieutenant who heads the organization, said. While the unit has existed for years, he said "it has taken a more aggressive stance under this administration."

"No comment" was the official response to why an unarmed man with no police record was shot at all, and why so many times.

Meanwhile, Mayor Giuliani, who said he welcomed Federal involvement, appealed for calm and patience until the facts were in. He said that the police officers all had good records and that the victim, a street peddler who worked on 14th Street in Manhattan, had no police record.

Standing with Police Commissioner Howard Safir at a City Hall news briefing, the Mayor said, "It obviously troubles both the Police Commissioner and me that 41 shots were fired. We don't know the reason for it at this point, but that is what the investigation is all about."

Commissioner Safir said there were no eyewitnesses to the shooting, other than the four officers. Asked why Mr. Diallo had been approached by the police, he said, "There are some similarities between the individual and the sketch we have," a reference to a rapist being sought, "but beyond that, what was in their mind, whether they thought this was that individual, is unfair to speculate at this time."

The officers — Edward McMellon, 26, Sean Carroll, 35, Kenneth Boss, 27, and Richard Murphy, 26, all members of a street crimes unit composed of some of the city's most aggressive and decorated officers — were on administrative leave, and had not been questioned by investigators for their department or the Bronx District Attorney's office.

Thus, no comment was the official response to the unanswered questions: why an unarmed man with no police record was shot at all, and why so many times.

Some law enforcement officials said the phenomenon of several officers' firing many shots is sometimes called contagious shooting. In the heat of what one officer believes is a deadly confrontation, the first shots may be justified, but others join in as a kind of contagion.

Mr. Diallo was, by all accounts, a pious Muslim who had never been in trouble with the law.

James Savage, president of the P.B.A., called the shooting a "terrible tragedy."

He added, "We do not have enough of the facts yet to be able to comment in substance about how it occurred." He said the officers had not invoked the 48-hour rule, an often misunderstood contract provision that allows those facing departmental charges two business days before talking to investigators. Departmental charges usually are not pursued until after criminal charges are adjudicated.

Regarding criminal charges, officers, like other citizens, may invoke constitutional rights against self-incrimination. The four officers in this case have not been questioned by police investigators at the behest of the Bronx District Attorney, who appeared to be proceeding cautiously to avoid legal errors.

The involvement by the United States Attorney does not mean that Federal officials will take over the investigation, officials said. Rather, Federal officials will consider possible violations of Mr. Diallo's civil rights, while the District Attorney examines criminal charges.

Typically, the Federal Government does not enter a case until later, often only after state prosecutors fail to win a conviction. That is what happened in the case of Officer Livoti, who was acquitted of criminally negligent homicide in 1996, but was convicted last year by Ms. White's office of violating the civil rights of Mr. Baez by killing him with a choke hold banned by the police.

AFTERWORD:

Possibly the most divisive case in the history of New York made signifcant progress on March 31 when the District Attorney officially charged the four officers with two counts of second degree murder. The first count involves the intentional killing of a person. It does not require premeditation. The second count accuses them of "depraved indifference to life," requiring the prosecution only prove the defendants recklessly caused Diallo's death. Each count carries a maximum prison sentence of 25 years to life.

The four were also charged with reckless endangerment in the first degree for endangering building residents by firing into a public vestibule. The charge carries a maximum of seven years of prison.

Meanwhile, Johnnie Cochran, the attorney who successfully defended accused wife-murderer OJ Simpson, announced that he and Barry Scheck, also from the Simpson team, were preparing a civil lawsuit on behalf of the Diallo family. Both criminal and civil proceedings promise to be trials to watch.