Brandishing AK-47s, they pulled everyone they could find from their beds - 21 people all told, including eight children and a pregnant woman - and forced them to lie face down on a concrete patio.
Then, systematically, they shot them all, the convulsing bodies piling on top of each other in a tight circle.
By the time the police arrived more than a hour later, the concrete was thick with congealed blood and littered with mounds of cartridge cases. Overturned furniture and scattered piles of children's toys showed mute evidence of a brief, hopeless struggle.
Many of the adults were wearing blue pyjamas. Two of the children were in nappies.
Moans led police to one adult and a 12-year-old still alive in the heap of corpses. A police jet rushed them to hospital in Mexico City. A third survivor, a 15-year-old girl, escaped unwounded because she hid under a bed. She was in hospital last night with severe shock. Armed guards surround her and police hope she may recover enough to be able to identify the attackers.
Neighbours who heard the automatic weaponfire believed at first they must be some kind of Independence Day firework display.
Yesterday Baja California seemed stunned, numbed by the worst mass killing in its history.
The region, a narrow strip of land that adjoins the US state of California, has crime problems - notably cross-border traffic in drugs and illegal immigrants, with sudden bursts of random violence that invariably accompany such activities.
But the seaside town of Ensenada, 60 miles south of Tijuana and San Diego, is better known for its vineyards, fine beaches and the weekend daytrippers who come by boat or car from the US.
"Ensenada did not deserve this," said the town's mayor, Manuel Montenegro Espinoza. "Nothing justified this act... It is something we have never seen before and must never allow to be repeated."
The massacre was an atrocity far beyond the standards of Mexican organised crime groups because of the deliberate killing of children.
Within two hours, army troops had surrounded the compound while local police and more specialised investigators tried to make sense of the carnage within.
Suspicion immediately fell on possible links between 38-year-old Fermin Castro, the owner of the Vista al Mar ranch, and Tijuana's Arellano Felix cartel, which is believed to mastermind marijuana production and shipment in the region.
Castro has been the target of intense police investigation for a year.
One report said the gunmen tried to force Castro to reveal the number of a bank safety deposit box in the United Arab Emirates where they suspected he was collecting a fat offshore account in hard currency.
As the story went, when Castro refused, his entire extended family paid the terrible price. Castro was not believed to have died instantly, but his name appeared on the list of the deceased within a few hours.
Law enforcement officials were cautious. "We cannot say one hundred per cent that there was a drugs trafficking angle or that this was a settlement of scores. We just don't have enough information yet," said the state prosecutor for the northern part of Baja California, Marco Antonio de la Fuente Villarreal.
"What we can say is that we have various different leads. We are questioning a number of people who live near the ranch to try to harden up the motive for this lamentable act."
Whatever Castro's more underhand activities, he was a well-known figure around Ensenada, organising rodeos for special occasions and he was an enthusiastic breeder of horses. Many of his animals were grazing around the ranch when police arrived.
The most popular line of speculation was that Castro, pictured in newspapers as a rotund figure with a long droopy moustache, was involved in marijuana production and fell out with the Arellano brothers in Tijuana.
All four Arellanos have been in hiding since the assassination of a Catholic cardinal at Guadalajara airport in 1993. The cardinal is thought to have been mistakenly shot by an Arellano hit man who was aiming for a drug cartel rival.
Local officials seemed less preoccupied with the organised crime angle than with the possible impact on the all-important tourist trade.
The governor of Baja California, Hector Teran Teran, said the massacre at the ranch was "bound to have a negative effect".Reuse content