American officials here acknowledge that the mayhem is unpredictable but contend that they have a way to help tackle it, spreading word that the $1.6 billion Merida Initiative, Washington ’s signature antidrug program, will step up training and advising for the Mexican state and local police and judicial institutions this year, rather than emphasizing the delivery of helicopters and other equipment.

In a year in which President Felipe Calderón ’s party, in power since 2000, may struggle to hang on to the presidency in July elections, the expanding violence is giving political rivals, all promising a more peaceful country, much to run on.

Discerning patterns of violence in the drug war can be perilous; it is often like a tornado skipping across terrain, devastating one area while leaving another untouched.

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But government statistics released last week showed a surge in deaths presumed to be related to drug or organized crime in Mexico State, which surrounds the capital and is the nation’s most populous state, in the first nine months of last year. The government data also show that violence has now afflicted 831 communities nationwide, an increase of 7 percent.

Although questions have emerged about the government’s tally, many analysts agree that the violence is widening.

“There has been a definite shift of violence away from the border and back to the interior states,” said David A. Shirk, director of the Trans-Border Institute at the University of San Diego, who closely tracks drug crime.

In a way, he said, the shift is a stark reversal of the trend of six years ago, when violence exploded in more southerly states and migrated north along drug-trafficking routes, accelerating a drug war that has now left more than 47,000 people dead, according to the government.

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In response, the Mexican government deployed its military and the federal police, arresting and killing more than two dozen cartel leaders and splintering or dismantling several groups. Their push has been backed by American aid in the form of helicopters, remotely piloted drones and the deepening involvement of American drug agents in investigations and raids.

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The violence slackened in many areas along the border, including Ciudad Juárez , the bloodiest city, where homicides have been declining. Mexican officials say the decrease is proof that they are making headway, but analysts say it may have more to do with one rival group’s defeat of another, reducing competition and the bloodshed that comes with it.

As for the violence in other areas — Acapulco , in the south, is now the second most violent city — that, too, may reflect the shifting contours of the fights between criminal organizations.

The drug war, Mr. Shirk and other analysts say, is increasingly coming down to a fight to the death between the Sinaloa cartel, a more traditional drug-trafficking organization widely considered the most powerful, and Los Zetas, founded by former soldiers and considered the most violent as it expands into extortion, kidnapping and other rackets in regions far off the drug route map. A third, the Gulf Cartel, remains well armed and rises to attack from time to time.

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Many of the clashes have been in central or more southern areas where the two main rivals have not previously fought each other so violently, analysts say. George W. Grayson, a longtime researcher of Mexican violence and co-author of a coming book on Los Zetas, said the group had spread to 17 states from 14 a year ago.

Though experts have said that Mexico City’s size, complexity and police force, considered better trained than many others, make it unlikely to fall into the mayhem of other locales, there have been alarming signs that violence is encroaching on the capital.

At the mall where the bodies were found, a banner proclaiming it was the work of the Sinaloa cartel appeared nearby, though experts say the killings could have been carried out by any number of offshoots operating in the region.

The murders were not the first in or near the capital to bear the signature of a cartel; in October two human heads were found on a busy road near the Defense Ministry headquarters.

But as the government, buttressed by United States drug agents and military advisers, deploys its armed forces and the federal police to dismantle criminal organizations and causes them to splinter, it has grown difficult to determine which criminal group is doing exactly what.

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The conflict has undergone “Zetanification,” as all manner of criminal outfits copy the cartel’s brutal tactics and claim its name, said Mr. Grayson, a professor at the College of William and Mary.

Mexican officials continue to assert that they are getting the upper hand. In Washington last week, Mexico ’s public safety secretary, Genaro García Luna, warned that violence would probably not decrease significantly for five more years. But he insisted that progress was being made, saying the rate of increase in homicides believed related to organized crime was showing signs of slowing. “You have to give the process more time to measure its efficiency,” he said.

At the mall in Mexico City, in the high-end Santa Fe district, known for its financial buildings and apartment towers, shoppers said they were worried but growing accustomed to gruesome violence in the country.

“We are living in a terrible situation,” said Jasia Grinberg, 65, who runs a hair salon at the mall, Centro Santa Fe, “and meanwhile, we are getting used to it.”