LATIN AMERICA : Nightmare for Brazil: Killers May Wear Badge : Gangland-style slayings shock the nation. Even worse, mounting evidence points to the military police. 21 died in the most recent massacre.

Hundreds of mourners from Vigario Geral, a slum west of here, marched through a suburban cemetery recently. They bore the simple wooden coffins of a dozen victims of a gangland-style slaying that stunned this country and the world.

It was a tragic scene that has become all too familiar here. In this continent-sized country of bitter contrasts, where poverty and plenty rub shoulders, violence and grief have become sadly routine.

In July, eight street children were fatally shot by gunmen as the youngsters slept near the stately Candelaria church in downtown Rio. Then the butchery in Vigario Geral, a poor community of 30,000 lying along the railroad in Rio’s west zone, shocked even those hardened to the ways of Brazil’s surliest city.

Around midnight on Aug. 30, 40 men wearing blue jeans and black hoods stole across a narrow pedestrian bridge that spans the tracks and poured into the tangle of footpaths in the area. They carried Uzis, 12-gauge shotguns and even an AR-15 assault rifle.

For the next hour, they spread a trail of terror. When it was over, 21 people were dead. The victims were students, porters, mechanics and retirees. One was a 15-year-old girl.

A stunned man muttered the same phrase repeatedly, as he gazed at the coffin of Juacyr Medeiros, his former friend and drinking buddy who was slain in his bar by the masked gunmen. “Never have I seen anything like this,” the man said.

Nor had anyone else.

Decades ago, a pioneering sociologist described the Brazilian as the New World’s quintessential “cordial man,” a conciliatory soul whose manner is as winning as the tropical breeze. The moniker grew into a deep-rooted, widely held belief about the national character.

But Brazilians are awakening to another, less flattering image--that behind the “cordial man” there is a killer. Worse, the killer may wear a badge. Mounting evidence has tied the recent bloodletting to the very people whom society has charged with keeping the peace: the police.

Authorities here say the massacre was a brutal act of revenge for the killing, just a day before, of four patrolmen in an ambush by heavily armed men; those who killed the police reportedly worked for local drug lords.

Leonel Brizola, the Rio state governor, and Nilo Batista, the lieutenant governor, both officially have conceded that the revenge killings were the action of rogue military police.

In fact, five military police were arrested Friday as suspects in the massacre after investigators found them with a cache of high-caliber weapons and 17 black hoods, such as those witnesses said were used in the attack on Vigario Geral.

“The only solution is to call in the army,” said Antonio, who lives in the area and spoke at the residents’ association headquarters. “We have no protectors.”

Created during the Brazilian dictatorship, the military police corps--”PMs,” as they are known--have created controversy wherever they have trod. Four PMs have been arrested in the July slayings of the eight street children in downtown Rio; 38 others already have been jailed for roles in “death squads.”

Police violence forced Brasilia to send army troops into Alagoas state and prompted the governor of Amazonas state to dissolve its military police corps. Last October, military police in Sao Paulo entered the Carandiru prison to break up a riot and killed 111 inmates. Last year, a record 1,470 civilian deaths were attributed to Sao Paulo PMs, a number the human rights organization Americas Watch called “a catastrophe.”

Scholars have blamed the violence on everything from brutal training procedures to meager pay, which encourages patrolmen to moonlight as “exterminators.”

But impunity is perhaps the chief culprit. Almost without exception, PMs charged with crimes go on trial in military courts, where punishment rarely goes beyond suspension. Almost a year after the Carandiru massacre, two officers were relieved of command but 10 others have since been promoted; none has been jailed.

While Brazilians are still trying to shed the shock of so many tragedies, authorities in Rio have been pressured sufficiently to announce an overhaul of the military police; 58 PM colonels will be removed.

Half a dozen legislators have demanded federal intervention in Rio. Brasilia rejected that. Justice Minister Mauricio Correa has created a unit to investigate large-scale killings in which police are involved.