The deed was stomach-turning: Hooded gunmen burst into a Ciudad Juarez drug treatment center, gathered together those inside and lined them up before opening fire with semiautomatic weapons. When the shooting was over, 18 people were dead.
Attention focused immediately on the site of Wednesday night's killings: a rehab center, where addicts go to get clean, suggesting a new level of depravity in Mexico's drug violence.
Theories abounded: The victims were targets of rival gang members. They owed money to the wrong people. They were pawns in a turf war between cartels that has made Ciudad Juarez the scene of a bloody death match for 20 months.
Odds are that the slayings, like hundreds of others in the border city, will never be solved. The crime is a further sign of the chaos enveloping Ciudad Juarez and a reminder of another tragic development that has accompanied the flow of cocaine and other drugs through Mexico: a big and growing problem of local drug addiction.
What was remarkable about the rehab center killings was how unremarkable that sort of violence has become in the city, which has seen about 3,000 violent deaths since the start of last year. True, the attack stood out for the spot where it took place, and the toll was higher than the usual daily tick-tick of slayings in ones, twos, sixes, 10s.
In the previous week, at least 75 people were killed in the city, including a man who was beheaded, another suspended by handcuffs from a chain-link fence and four whose bodies were piled on a sidewalk.
Those killings went largely unnoticed outside Ciudad Juarez. And there was little fanfare last week when the Mexican army announced the arrests in the city of three men it said had confessed to killing 211 people. It provided almost no details on the allegations.
The clinic killings, which President Felipe Calderon labeled "dramatic and terrible," underscored Mexico's emerging struggle with drug abuse. Mexican leaders say some of the country's escalating violence is connected to growing domestic consumption, which is sparking turf battles over local markets. Once merely a pipeline for narcotics bound for the United States, Mexico is now grappling with its own problem of drug use and addiction.
"Criminal activity went from being low profile and non-intrusive in the lives of citizens to being defiant and, particularly, violent," Calderon said in his state of the nation speech Wednesday.
"The search for markets for consumption in Mexico has spread practically throughout the whole country," the Mexican president said, defending his government's 33-month-old offensive against drug traffickers.
Government data show that addiction rates here have risen quickly as residents experiment with relatively cheap versions of cocaine and methamphetamine. It has gotten easier to find drugs on the street in Mexico because tighter U.S. border enforcement has made it harder to move them north, some experts say.
A government survey released last year found that more than 460,000 Mexicans were addicted to drugs, a 51% increase from six years earlier.
In response, thousands of clinics have sprung up around the country, many of them small fly-by-night operations that are largely unregulated.
The Ciudad Juarez clinic, a converted house called El Aliviane and one of dozens of such centers in the city, sits in a neighborhood next to the border that is plagued by gangs, prostitution and drug use. On Thursday, the floor of the pink-painted house was coated with blood.
The attack followed assaults on at least four other rehabilitation clinics in the city during the last 13 months, according to news reports. In one attack last year, gunmen killed eight patients and wounded six.
Victor Valencia, public security secretary for the state of Chihuahua, said 20 people were in a meeting room when the attackers burst in. The gunmen ushered them into a central patio and opened fire with AK-47 assault rifles, he said. Investigators found at least 80 spent casings. Two of the victims were wounded but survived.
The father of Jaime Saul Perez, a 17-year-old who was slain, said his son had finished eight months of rehab but continued living at the center to attend prayer meetings.
"He was getting out," said Jaime Perez, the father. "He promised me he was going to change."
Valencia, interviewed on Mexican television, said the slayings may have stemmed from a dispute between rival criminal gangs. El Diario newspaper reported that a number of the dead were members of a well-known gang called the Aztecas.
Alberto Islas, a Mexico City-based security specialist, theorized that the slayings were in retaliation for a weekend shooting that killed eight people at a street party in the neighboring state of Sinaloa.
"We are entering a new dimension of terrorist attacks between cartels," Islas said.
Others said the Ciudad Juarez attack was the latest episode of killings of young members of street gangs who use or sell drugs. Mayor Jose Reyes Ferriz told The Times several months ago that the death toll in his city was rising fastest among youths trying to break into the street trade in drugs.
Tony Payan, a border scholar at the University of Texas at El Paso, said the victims may have owed money to suppliers or been hit by rivals because they remained involved in the drug trade under the cover of the treatment center.
"They're after particular people," Payan said. "In the end, the people who end up in these centers are involved in the business."
Drug treatment centers in Ciudad Juarez and elsewhere in Mexico draw some clients from street gangs that serve as foot soldiers for drug cartels, particularly two rival groups based in the city and in Sinaloa. Gangs often use the facilities as recruiting grounds, creating potential targets for enemies.
Ciudad Juarez, a city of 1.3 million, has for more than a year been the scene of the worst violence in Mexico amid the government's war against drug traffickers.
More than 11,000 people have been killed nationwide since Calderon launched the crackdown in December 2006. Most of the killing is a product of fighting between drug rivals over control of coveted routes for smuggling drugs to their main destination, the United States.
Calderon has mobilized 48,000 troops and 5,000 federal police in the nationwide offensive. But despite the deployment of more than 9,000 soldiers and police to Ciudad Juarez alone, the bloodshed continues there, stemming from a variety of forces: rival cartels, conventional street gangs and small-time crooks, dirty cops and the government crackdown.
"It's a free-for-all," Payan said.
"You have a very chaotic situation."