MEXICO CITY — "Look, we need help!" the cop shouted into his cellphone in the middle of an enraged mob. "We're in Tlahuac, and the people are beating us up. Please send backup!"
It took riot police three hours and 35 minutes after Edgar Moreno called to get there. In a widely televised breakdown of social order and government authority, hundreds of citizens turned against a trio of federal undercover agents late Tuesday, apparently mistaking them for kidnappers, and burned two of them to death near their overturned car.
Mexicans agonized Wednesday over the country's most shocking recent episode of vigilante justice. But in the mean streets of Tlahuac, on the southern edge of the capital, few voiced surprise that the police had been so slow to respond.
"It happens to us all the time," said Daniel Flores, a 21-year-old engineering student who said he stumbled upon a major gang fight in his neighborhood a few months ago but could not get the police to come break it up. "Sometimes it takes days for them to respond to an emergency call, if they respond at all. Many people don't even bother to call them.
"Law enforcement is a joke," he added.
Wednesday was an exception. More than 500 federal and city police officers swept into the community like an occupation force. By evening, 20 suspects in Tuesday's killings had been seized in raids on their homes, the authorities said.
Vigilantism has been rising across Mexico in step with public disgust over the government's inability to stop violent crimes. After a quarter of a million people joined an anti-crime march in downtown Mexico City in June, President Vicente Fox promised a $100-million increase in crime-fighting efforts by year's end.
But there is little evidence of improvement. Police officials Wednesday offered a worn refrain to explain why only a few policemen initially responded to the mob violence: There are not enough officers on the force.
"It is very difficult," said Adm. Jose Luis Figueroa, chief of the Preventive Federal Police, whose officers came under attack. "We do not have enough coverage across the country to act immediately everywhere."
Twelve police officers are supposed to protect the 35,000 residents of San Juan Ixtayopan, the community in Tlahuac where Tuesday's violence occurred. To help people feel safer, local authorities have distributed dozens of crude bullhorn alarms, equipped with flashing red lights.
As dusk fell Tuesday, those alarms sounded from rooftops, summoning residents to confront what they thought was a threat to their children.
A gray Ford Focus sedan occupied by the three plainclothes agents had parked half a block from the Popol Vuh primary school, where 380 children were finishing afternoon classes. One of the men in the car turned on a video camera.
It was not the first time in recent weeks that mysterious men had been seen riding through the community and filming, especially near the school. Parents who gathered at the school Wednesday said they believed that would-be kidnappers had been stalking their children to find out who lived where.
In fact, police officials said, the three agents were staking out a house near the school thought to be a haven for small-time drug dealers. San Juan Ixtayopan, a settlement of small concrete-block homes at the foot of a snowcapped volcano, is home to factory workers and small shopkeepers but also a growing number of gang members who sell marijuana and crack cocaine. Police said two dozen callers had denounced the alleged drug safe house.
"Two weeks ago we complained to the authorities about these men in cars, but nobody ever told us it was the police," said Maria Luisa Cordoba, 40.
On Tuesday, a rumor -- unfounded, it turned out -- spread that two children had been abducted and driven off in a taxi. When the three agents parked near the school, suspicion fell on them as accomplices in the imagined crime.
A crowd quickly surrounded the car and swelled to nearly 2,000 people. They cheered, chanted and shouted obscenities as they pulled the plainclothes agents from the vehicle, overturned it, and manacled and pummeled their victims.
"They were kicking them and beating them with metal pipes, and blood was everywhere," said Ana Laura Juarez, a fifth-grader who was frightened by the violence but ran home to watch it on television.
Mexican television crews had arrived and were allowed to interview the bloodied, captive agents, who insisted that they were policemen. Television reporters at the scene said the crowd either did not believe the men or suspected that they were corrupt officers involved with a criminal gang.
"People tell us you got in a taxi with two girls," a reporter said to Moreno on Televisa's nationwide broadcast.
"That's a lie," the policeman replied, before the cameras recorded his distress call.
That was at 6:10 p.m. About 8:30 p.m., according to a chronology published by Mexican newspapers, the mob doused police officers Victor Mireles and Cristobal Bonilla with gasoline and set them afire.