The four men in bulletproof vests, Kalashnikovs held casually at their sides, crossed the street to Tijuana's Crazy Banana pool hall so calmly that onlookers presumed they were undercover police officers – until they heard the gunfire and screams.
Moments later, the men raced back out of the bar and sped off in a getaway car, leaving the once-popular pool hall with its thatched roof and yellow painted walls a bullet-ridden crime scene.
The five billiards players gunned down there were the some of the latest victims in a brutal drug turf war that has unleashed an orgy of killing along America's southern frontier.
The attack was one of dozens of recent incidents in the sprawling Mexican border city, where nearly 300 people have been killed since late-September – many mutilated, tortured and beheaded in gruesome terror tactics copied from Iraq's brutal conflict.
In the past week alone, there has been an attack in a nightclub popular with students that left five young people dead or dying; a hit squad stormed a private hospital and killed a patient who was being treated for gunshot wounds; and armed men opened fire on a car parked outside a popular US-owned discount warehouse, killing a woman and seriously injuring a man.
Mexico's drug war death tally of more than 4,000 this year – 685 in Tijuana – makes it one of the most dangerous countries in the world, and the extreme violence has intensified since the federal government launched a crackdown against the cartels.
On streets in the centre of Tijuana, where throngs of American visitors once stocked up on cheap goods and prescription drugs by day and revelled in the brash nightlife after dark, stores and bars now stand empty.
"Help the Mexican economy. Visit my shop!" pleaded one souvenir store manager last week as an armoured vehicle carrying heavily-armed troops trundled past him, down Avenida Revolucion.
But there were no shoppers for the T-shirts and spirit shot glasses bearing the legend, "One tequila, two tequila, three tequila, floor" – Tijuana's unofficial slogan for the young Americans who used to pour over the border to evade America's 21-year drinking age.
Nearby is the Caesar's restaurant where in 1924 an Italian immigrant, Caesar Cardini, devised a tangy dressing to accompany leftovers of lettuce and bread crusts and thereby invented the eponymous salad. But waiters stood around forlornly in the absence of customers.
After incidents like the Banana Loca killings, that is hardly surprising. Investigators suspect that the hitmen who calmly marched into the bar were on the trail of rival gang members, but the indiscriminate gunfire claimed the lives of innocents, including 23-year-old Jazmin Martinez, a visitor from Ciudad Juarez who was relaxing after taking part in a women's indoor football tournament.
Just a day earlier and 100 yards away, shopowner Miguel Angel Cepeda was cut down in hail of bullets from gunmen trying to wipe out a senior police officer who escaped unhurt inside his store.
Such is the mood of terror just 15 miles south of the laid-back affluence of San Diego that mourning relatives and eye-witnesses were too scared to talk to the media, even anonymously.
"Welcome to our world," said one business owner who had ducked for cover twice in two days. "Say the wrong thing, see the wrong thing, be in the wrong place and you are a dead man."
A security guard at a nearby mall reluctantly acknowledged that he saw the hit men before they struck at the Crazy Banana - but mistook them for undercover police.
"The four men get out of a car and walked calmly across the road," he said. "They were wearing armoured vests and carrying Kalashnikovs and walked into the bar. I thought they were police until the gunfire erupted."
Although most murder victims are cartel foot-soldiers, the shocking paroxysm of violence has had a debilitating impact on the rhythms of life for everyone in Tijuana, a rough and tumble city of 2 million immediately south of the border fence.
"I go from home to work and back home, I lock the door behind and I stay home," said Ana Sanchez, a secretary. "My husband and I take turns to drive the kids to school in the morning and bring them home in the afternoon. If they want to visit friends, we drive them. And we don't let them play out because we're scared of crossfire."
Increasingly the gangs are seeking to outdo each other in their barbarity as they fight for power and try to strike terror into their rivals. The dead have often been tortured first before being dispatched. Mass graves are commonplace.
"They're excelling themselves in finding ever more creative ways to kill each other," said Victor Clark Alfaro, a veteran human rights activist and expert on the cartels. "Violence is nothing new in Tijuana. What is new is the shocking level of violence."
The brutality in recent weeks has two causes. First, a battle is now under way for control of the world's most lucrative border smuggling corridor, that funnels Colombian cocaine, Asian heroin and Mexican-manufactured methamphetamine to US markets. This part of the frontier was long the terrain of the Tijuana cartel run by the Arellano Felix family, the country's first narco-clan. But a spate of arrests and killings means that just one of the 11 siblings who inherited the cartel from their uncles is still involved in the leadership: drug "queen-pin" Enedina Arellano Felix. While she supervises the financial and money-laundering business, her nephew Fernando "The Engineer" Sanchez Arellano is in charge of smuggling and armed operations.
The weakened family business, however, is under siege from the rival Sinaloa cartel, Mexico's most powerful drug gang. And in this internecine war, some of the Tijuana organisation's most-feared figures have switched sides, throwing in their lot with the Sinaloa faction. They include a ruthless killer, known as El Teo, and his feared lieutenants known as Crutches (because he has left many victims crippled) and The Bitch (a man, despite the offensive nickname).
As the two cartels pursue their blood bath, the Mexican government under conservative president Felipe Calderon has finally taken action. It has flooded the region with federal police and troops to take on the narco-barons, after two decades during which the government's war on drugs has been crippled by corruption and incompetence.
The move has sparked an even more violent response from the cartels.
"This is the price we will have to pay to clean the city," said mayor Jorge Ramos. "In cleaning our house, we are going to get dirty."
A key part of the government crackdown is to crack down on parts of the government itself. In Tijuana two weeks ago, 500 police were taken off the streets to undergo mandatory retraining and tests – including polygraph and toxicology – to try to assess whose side they are actually on. For the time being, they have been replaced by Marines and federal officers.
Earlier this month, 21 senior police officers in the city were suspended or arrested for suspected collusion with organised crime. And only last week, Mexico's top anti-drugs official, Noe Ramirez Mandujano, was arrested on suspicion of selling information about his findings to the very people he was supposed to be investigating.
His detention came just four days after that of Ricardo Gutierrez Vargas, a senior federal police chief and Mexico's top liaison official with Interpol, was put under house arrest, also on suspicion of leaking secrets to the drugs lords.
Alberto Capella Ibarra, the Tijuana city secretary for public security, knows all about the risks involved in taking on the cartels in his violence-plagued city. Three days before he even took over the police force last December, he had to fight off would-be assassins with an automatic rifle when they attacked his home.
The former corporate lawyer urged his fellow residents not to weaken their resolve in the face of the cartel savagery. "This is very painful for our city but at some stage we had to reclaim Tijuana," he said. "We have decided to fight and that has prompted this violent reaction as a backlash. But we cannot give up now."
The number of empty homes in affluent neighbourhoods in the hills above the city is evidence that many wealthy residents have already given up, however – scared away by the drug violence and threat of kidnapping for ransom.
"People would rather leave their home and country, and move to San Diego to be safe, than to live like this," said Cristina Palacios Hogoyan, 68, a member of well-to-do Tijuana business family whose son, Alex, was snatched 10 years ago and never seen again after informing on friends in the drug trade.
She heads an association that represents families of the "disappeared" and last weekend organised an anti-violence march through the city's streets. Some had apparently lost hope that man could deal with the challenge – the most common placard read simply: "God save us."