The killings began before dawn. Men armed with automatic rifles walked through the hillside slum of Grand Ravine, warning of a fire and yelling for residents to come out of their cinder-block and sheet-metal shacks. Those who obeyed were gunned down.

Several hours later, Haitian morgue workers and UN peacekeepers from Sri Lanka piled bodies in one of the slum's main thoroughfares, a rocky stream bed at the bottom of the ravine after which the neighbourhood is named. The body count totalled 21, including three women and four children. Most of the victims were killed with a bullet to the head.

Yves Jean-Philippe, a 56-year-old street vendor, was found in a dirt courtyard, his eye socket ripped apart by a bullet. Alnosia Desir, wife of a Christian pastor, was shot in the mouth and throat in her bedroom. The body of Jean Willerme Sanon, 12, lay face down on a twisting pathway, his head split in half.

'What is shocking is that all victims appear to have been innocents. We're talking about women and little children - these were no bandits,' said Jean Gabriel Ambrose, the Port-au-Prince JP whose job is to verify the names and ages of victims of violent crimes, along with the cause of death, before the bodies are taken to the morgue.

The massacre was as unexpected as it was gruesome. For several weeks, rival gangs had exchanged fire in a turf war over control of the slum. But the massacre that took place last Friday was so arbitrary - family members, neighbours, human rights observers and police all agree the victims were not gang members - that UN and Haitian officials believe it may have been in part an attempt to destabilise the newly elected government of President Rene Preval.

'I don't believe it was a spontaneous attack,' said Desmond Molloy, who heads the UN's disarmament, demobilisation and re-integration programme in Haiti. 'This massacre creates an atmosphere of fear and, when people are afraid, it's very hard to establish any degree of stability.'

The killings in Grand Ravine have shattered five months of relative peace that had followed Preval's landslide victory on 7 February. The election marked the first sign of improvement after two years of severe crisis and violence that followed US Marines whisking former President Jean-Bertrand Aristide into exile in February 2004.

Preval came to power supported by many members of Haiti's tiny but powerful elite. The daily firefights between UN peacekeepers and armed groups loyal to Aristide in the sprawling slum of Cite Soleil stopped, and a surge in kidnappings that had panicked foreigners and Haiti's small number of middle and upper classes abated.

In Grand Ravine and the neighbouring slum of Martissant, opposing gangs made peace during a 19 March football match sponsored by the UN. But the truce did not last long. 'In recent weeks, we'd been aware of a heightening of tensions among the gangs along political and territorial lines,' said Molloy.

On one side was a gang based in Grand Ravine associated with Aristide's Lavalas party. On the other were two allied gangs in neighbouring slums, one based in an area called Ti Bwa, while the second was opposed to Aristide and called the Little Machete Army. The latter earned its name at another football match in Martissant in August 2005 that ended in bloodshed when police officers began shooting in the stadium and the machete-wielding gang hacked to death the fleeing spectators.

Both residents of Grand Ravine and Haitian government officials blame the Little Machete Army and the Ti Bwa gang for the massacre last Friday. What remains a mystery is what provoked these gangs to murder more than 20 innocent people.

Haitian police chief Mario Andresol suspects the attack is related to the killings at last year's football match, which appeared to be a joint effort by the Little Machete Army, backed by rogue police officers, to eliminate the Grand Ravine gang. Andresol arrested 15 police officers for their alleged participation in the stadium killings, but the judge handling the case has since released most of them, including two senior officers, Renan Etienne and Carlo Lochard.

Some residents of Grand Ravine accuse Lochard of reuniting with the Machete Army since his release. 'The same police officers who made the alliance with the Machete Army are the ones who helped commit the massacre,' said Joseph Albert, an unemployed resident of Grand Ravine. 'Lochard has given them guns and money.'

Andresol was confirmed by the senate to continue his term as police chief the day before the massacre occurred, leading some observers to speculate that the killings represented a warning to him.

Since the massacre, Sri Lankan peacekeepers have so far managed to ward off more violence. But dozens, perhaps hundreds, of the area's poor residents have fled anyway.

The UN and Haitian police have launched an investigation into the massacre, but hopes of identifying those who pulled the trigger, not to mention those who provided the guns, remain dim.

'This is my 13th conflict, and it's been the toughest one to find out what's really going on,' said Molloy, a former Irish army officer who headed the UN's disarmament programme in Sierra Leone before coming to Haiti in 2004. 'It's very difficult to nail down the motives behind actions in Haiti and there's often a mix of political, economic and territorial motives at play.'