Brazil's justice system, long criticized as corrupt and inefficient, has again touched off national and international furor following a jury verdict last week that exonerated three police officials accused in connection with the slaying of 19 landless workers in 1996.

Human rights activists, government officials and members of Brazil's landless workers movement had been optimistic about a guilty verdict--unusual in a country whose police, known as some of most venal and violent in the world, are rarely punished for their crimes.

They had hoped that the trial would provide evidence that Brazil had made progress in grappling with police violence and corruption, particularly in isolated and poor regions such as Para, the northern state where the April 1996 slayings occurred.

A day after the jury's decision in a state court last Thursday, an angry President Fernando Henrique Cardoso took the unusual step of criticizing the verdict, saying it suggested that nobody was responsible for the killings. "It can't be that it was nobody," he told reporters. "It had to be somebody."

The lead prosecutor, scheduled to try another 150 officers who allegedly took part in the killings, quit the case in protest. Legal experts say that a not-guilty verdict in this first trial has made it virtually impossible to convict the remaining defendants.

"This is a case that speaks of an open wound in Brazilian history," said Nilo Batista, a prominent lawyer who helped the prosecution in the case. A guilty verdict "would have been an advance, but it wasn't."

Ney Strozake, a Brazilian human rights lawyer who works with the Movement for Landless Workers, known as MST, called the decision "a great shame for the Brazilian people. The message to the world is that Brazil doesn't respect human rights. Our police can kill, they can wound, and they never get punished."

Supporters of the acquitted officials stressed, however, that a majority of the seven-member jury had said in a court questionnaire that they did not think the prosecution had presented enough evidence to convict the three defendants. But prosecutors, who are appealing the verdict, contend that some of the questions asked by the court were inappropriate and were misunderstood by some jurors--and that jurors accepted payment from police in return for a favorable verdict.

The killings occurred when military police clashed with hundreds of landless workers who had decided to march to the governor's office to protest their plight. They were part of a movement that has become increasingly powerful in this sprawling country, where the wealthy and near-wealthy own 90 percent of the land.

After several hours of tense negotiations between the government and protesters, police were called in to smash the demonstration. They arrived with revolvers, rifles and automatic weapons; the workers, for the most part, carried only sickles.

Shots split the air, and when things had calmed 40 minutes later, 19 protesters lay dead. Some of them had been shot point-blank. Others apparently had been hacked to death with their own sickles.

No police officers were killed.

National and international outrage followed and grew stronger when it turned out that television cameras had caught the killings. "There was so much international and national pressure," said the Rev. Ricardo Rezendes, a Catholic priest who works with rural laborers. "I thought that at least to give the public some satisfaction, there would have been some kind of punishment, even the minimum."

During the trial, defense attorneys argued that the military police, outnumbered by the protesters, used their weapons only after a demonstrator fired first. Videotape of the incident supported their assertion.

But police investigators committed a slew of errors within the first days after the killings that ultimately made it virtually impossible to tell which officers did the shooting, human rights activists charge. Police didn't get witnesses to try to identify officers who were present, didn't check officers' hands and clothes for gunpowder residue and failed to secure information about the weapons officers carried, making ballistics tests impossible, the activists say.

"The investigation was a comedy of errors," said James Cavallaro, director of Brazil's office of Human Rights Watch. "The big problem with this case is that the police investigators and prosecutors knew very little. Police and prosecutorial authorities have done such a bad job that there was really little chance to figure out exactly who did what."

CAPTION: Members of Brazil's landless workers movement chant slogans while awaiting jury's verdict last Thursday in trial of three police officials in Belem.