By Steve Kingstone
BBC News, Sao Paulo
In May 2006 South America's biggest city was terrorised by a wave of brutal attacks by a crime network. The violence claimed the lives of 32 police officers, but police hit back with lethal force of their own. Six months on, evidence is mounting that officers summarily executed dozens of suspects, as Steve Kingstone reports.
The violence convulsed Sao Paulo for several days
Heliopolis is a vast maze of alleyways and winding streets which together form the largest favela - or shantytown - in Sao Paulo. Controlled by drug-traffickers and scarred by gun crime, it remains a no-go area for most of this city's residents.
But the people of Heliopolis say their community has been the scene of flagrant human rights abuses, which have been ignored by the state government. They accuse the police here of murder.
"They shot my husband five times. It was ridiculous, senseless," protests the widow of 24-year-old Rogerio do Carmo Pereira.
He was killed, along with his brother and a third man, on the night of Wednesday 17 May.
Officers had apparently raided the favela after reports that an armed gang was planning to attack a nearby police station.
"They were in an alleyway," says Rogerio's widow. "The police asked if he had a criminal record. He said he had, and they didn't ask anything else. He was against a wall, and they opened fire."
Betraying a quiet anger, she adds that her husband's criminal history consisted of a single arrest - for stealing cigarettes. She insists that, on the night of his death, Rogerio was unarmed and working as a security guard at a local car park.
The police log of the incident states simply that the three men were killed after resisting arrest. There is no evidence that officers were killed or injured during the operation, or that guns seized in the raid were found on the three victims.
Remembering Ricardo: His mother says her son was an innocent victim
"We have a lot of shots in lethal areas - so it's typical of an execution," concludes veteran crime analyst Ricardo Molina, poring over the autopsy report into Rogerio's death.
As part of an independent inquiry into policing in May, Mr Molina assessed 124 fatal shootings by officers, categorised bluntly in police records as "resistance followed by death".
He concluded that in 60-70% of cases the victims appeared to have been summarily executed.
"There are people there with 21 bullets in their body," says Mr Molina, "and victims with a bullet in a downwards direction, right in the top of the head. So you can't say it was a shootout. If the victim was running, there should have been more bullets in the legs and arms."
The police have said that in May they were fighting something approaching an urban war with the First Command of the Capital (PCC), a powerful organised crime network.
But at the height of the conflict, officers were killing 15 suspects a day, compared with an average of less than one fatal shooting per day in 2005. Twelve of the victims in May were later buried without being formally identified.
Members of the independent commission of inquiry, set up by the state assembly, complain that they have run into a wall of silence from the Sao Paulo state government, which is responsible for policing.
During May's violence, police stations came under attack
"When we requested information about internal police inquiries, neither the Secretariat for Public Security nor state prosecutors would provide information about what was going on," says Paulo Mesquita, a commission member and a university lecturer on crime policy.
"We have no information about how many officers have been disciplined - either administratively or in the criminal justice system."
But cases in which officers acknowledge shooting suspects are only half the story.
The Sao Paulo police ombudsman is also looking into 87 unsolved murders from May, where members of the public have voiced suspicions of police involvement.
Often, the methods of killing were reminiscent of the state-sponsored "death squads" of Brazil's military era.
On 15 May Ricardo Flauzino was shot seven times in the head and chest as he waited for his fiancee in Jardim Filhos da Terra, an impoverished neighbourhood on Sao Paulo's northern outskirts. The 22-year-old worked as a delivery driver and had no criminal record.
When the BBC covered the case in May, local people queued up to allege that Ricardo's killers were police officers, wearing hoods and dark shirts over their uniforms.
They alleged that the same officers had then removed their disguises and returned to the scene of the crime, ostensibly to investigate the killing.
But six months on, those previously confident witnesses have melted away - too frightened to put their evidence on record.
"Sadly, the fear was too great," says a tearful Iracy Flauzino, the victim's mother.
"All these people said they saw something, but when I needed them they had forgotten or moved away. When the time came, they went back on what they had said. And I understand - we all have families."
The state government's Secretariat for Public Security turned down an interview request for this report, but a written response said there was no proof of any police involvement in the 87 unsolved murders from May.
It added: "There is no evidence which proves the existence of a death squad."
On fatal shootings by uniformed police, state officials say internal investigations are ongoing, but they will not confirm how many officers are being investigated or whether any officers have been disciplined or charged with a crime.
Without specifying cases, they say that a third of the fatal shootings in May were in response to "common crimes" rather than the PCC attacks.
The state authorities would offer no comment on Ricardo Molina's independent conclusion that 60-70% of the victims of police shootings showed signs of summary execution - other than to say that the analysis of autopsy reports was a matter for the police and judicial authorities.
"There is a huge lack of information and big discrepancies over the statistics, which is
frightening when we're talking about the rule of law," says Andressa Caldas of Global Justice, a leading Brazilian human rights group. "We won't shut up about this until we get answers."
Brazil's human rights community does not argue that all those killed by the police in May were innocent; nor does it deny that the wave of PCC violence was shocking and painful for law-enforcement agencies.
But campaigners allege that in May rogue officers handed down their own brand of street justice - mostly in poor, crime-ridden neighbourhoods where fatal shootings are only cursorily followed up.
"This issue had a high impact after May," says Andressa Caldas, "but it's important to emphasise that in many poor neighbourhoods people are still being killed. Of course some of them are criminals. But we have courts in this country, and that's where justice should be delivered."
Steve Kingstone's report will be broadcast on BBC Radio Four's Today programme on Saturday 18 November which is on air from 0700GMT to 0900 GMT.