Gareth Chetwynd in Rio de Janeiro

Sixteen-year-old Marcelo Julio Gomes do Nascimento admitted to a sneaking admiration for the police, and saw the respect that they commanded in his poor community with their tough-guy posturing and powerful weapons. Julinho, as he was known to his friends, was chatting on the porch of a shabby bar in the Rio de Janeiro suburb of Queimados, when the vicious reality of this image exploded in his face on a balmy evening last week.

An unmarked car swung into view, and its masked occupants unleashed a hail of bullets so accurate that barely a mark was left on the crumbling walls of the bar. But this was not a drive-by shooting carried out by petty criminals. Instead enough information has emerged to show that the killers were off-duty police officers acting out a private vendetta with their own bosses.

A short walk away from the abandoned bar, 33-year-old Adriana Paz Gomes sits on the steps of her modest home weeping for the loss of her son. "I was watching the evening soap opera when I heard the shots. In my heart I knew immediately what had happened," she recalls. Ms Gomes ran into the street and found four bodies in the road, blood pouring from lethal head wounds. Then someone pointed to a fifth body, that of her son. "I have such lovely memories of my son, but I can't remove the image of that hole in his head. He seemed to be sleeping and I took him in my arms, but he wouldn't wake up," she says.

Julio's death was the coup de grace of a bloody massacre that left 30 people dead in one night. The shooting began in the town of Nova Iguacu, where 18 people were killed. Some just happened to be in the street as the killers drove past, while nine of the victims, including three teenagers, were in a bar playing video games. The gunmen then moved on to Queimados where they killed 12 more.

Local people suspected members of the local military police force and Rio de Janeiro authorities quickly arrested 11 police officers and charged six of them. Off-duty police officers' participation in death squads is a sinister feature of the poor suburbs to the northwest of Rio de Janeiro, in a region known as the Baixada Fluminense. Here the homicide rate is 76 per 100,000, compared with 50 per 100,000 in metropolitan Rio, itself one of the highest rates in the world.

In some communities the role of vigilante groups is tacitly accepted as a way of reducing crime or preventing power slipping into the hands of the drug-trafficking gangs in the shanty towns. But last week's massacre was all the more sinister because most of the victims happened to be in the wrong place at the wrong time. The killers were apparently protesting against a disciplinary clampdown by a commanding officer recently installed at a local police battalion.

Two days before the shootings, two men, one of them a convicted drug dealer, were dragged from a bar and killed. A severed head was thrown into the compound in protest against the disciplinary measures. Uniformed officers were caught on film disposing of bodies, and eight were later arrested. Authorities have agreed that last week's massacre was a show of force by rogue police officers who opposed the arrests. "This was a group protecting its interests within a corporation and reducing people to symbols to send a message to their unpopular boss," said Pedro Strozenburg, a coordinator with Viva Rio, an anti-violence group.

President Luiz Inacio Lula da Silva has demanded swift action. "They say they are making arrests, but this is just for show. I know that one day I'll be seeing the man who killed my son driving past me in the street," Ms Gomes said.

The Guardian Weekly 15/04/2005, page 7