When does police mistake become murder?

It's late at night. An officer pulls over a car. As he approaches, he thinks he sees the driver reach for something. "Don't move," he yells. In the dark, it looks as if the driver is holding a gun. Partly out of fear, partly out of training, the officer opens fire. Seconds later, he discovers the man he shot was holding a cell phone.

Should the police officer be tried for murder?

Even when people are marching in the streets demanding justice, the answer for most juries is almost always "no." The law gives the police a lot of discretion in the use of force. And even in some shocking cases, juries often find there is doubt over what happened.

Last week, the issue surfaced again when the four Bronx police officers who pumped 41 bullets at Amadou Diallo, an unarmed African immigrant, were charged with second-degree murder. The case is likely to become yet another touchstone for debates over the police use of deadly force.

While there are about 600 to 700 police shootings per year, this type of high-profile case does not come up that often. "The closest thing to this in recent memory is the prosecution of the police accused of beating Rodney King," says John Crew, San Francisco-based head of the police practices division of the American Civil Liberties Union (ACLU).

And when one does, "it's quite hard to get convictions," says William Heffernan, editor of Criminal Justice Ethics, a journal published by John Jay College of Criminal Justice in New York.

The police officers accused of beating Mr. King were acquitted in state court, although some were convicted on federal civil rights charges. The four New York police officers may also face federal civil rights charges, which is one reason they decided not to testify before the grand jury.

The King case also shows how difficult it is to obtain reform, even after an egregious police action. The ACLU and other organizations tried to get legislation passed that would appoint a state prosecutor to pursue police abuse cases. Law-enforcement groups vigorously opposed it.

"We could not get that bill out of committee," says Mr. Crew. "The only bill related to Rodney King and the aftermath that was enacted increased the criminal penalties for rioting."

CRIME experts don't expect the Diallo trial to result in any new laws in New York either. The police unions have a powerful presence. And, as police experts are quick to point out, the New York Police Department has steadily reduced its use of deadly force, from killing as many as 90 suspects annually 30 years ago to fewer than 35 per year today.

Mr. Heffernan hopes the Diallo case will help put a stop to "racial profiling." "There must be a concern for minority misgivings about routine illegal stops and seizures and frisks," he says. "The key minority concerns are not about deadly force but about the indignities to their Fourth Amendment rights."

There is some indication that message is starting to get through. Mayor Rudolph Giuliani, after initially refusing to meet with minority leaders, has held several meetings. One result is the Special Crimes Unit - which includes the officers who shot Diallo - will wear uniforms instead of street clothes to make them easier to recognize. But Heffernan says police still need to show "greater sensitivity" to the minority community.

The police, however, are very sensitive about criminal charges brought against them. "The consensus of the cops and their lawyers is that they are scapegoats," says Larry McShane, author of "Cops Under Fire," which looked at cases where the police were involved in either shootings or incidents of brutality.

When questioning suspects, police like to be able to see their hands. In addition, Mr. McShane says, if suspects are not polite, "[police] feel their authority is being challenged."

This may have played a part in the Diallo shooting. From press reports, it appears the immigrant ignored police when they told him to stop. Instead, he started to go back into his apartment building. He apparently reached into his pocket, and police thought he had a gun. Instead, it was his wallet.

Last week, the FBI was using lasers to determine the angles of police bullets. There are some reports that police became spooked after some of their bullets ricocheted back at them. This may account for why they shot so many bullets at Diallo. The officers have yet to testify or give their version of the events.

James Fife, a professor of criminal justice at Temple University in Philadelphia, also points out that the NYPD's new 9 mm guns fire very quickly. "After they have used their weapons, lots of cops have no idea how many rounds they've fired," says Mr. Fife, a former New York cop.

Fife says the defense is likely to present enough arguments to make the case murky. "As a general rule, you must prove beyond a reasonable doubt that the police had not any reason to fear for their safety," he says.

But even if it ends in acquittal, Fife says the trial may have a restraining effect on other officers. "Every cop dreads shooting someone. There are so many questions that arise, and you wake up in the middle of the night in a cold sweat."