The recently released reports don't specify how or why the Americans were murdered, nor does it name victims. But 80 percent of them were killed in border states where narcotics violence is worst - 39 alone in Ciudad Juarez, which shares the Rio Grande with El Paso, and other nearby towns.

The impact on U.S. citizens visiting or living in parts of Mexico has steadily worsened since President Felipe Calderon deployed the army and federal police in late 2006 in an as yet unsuccessful attempt to crush the rising reach of the gangs.

The number of U.S. victims last year was more than triple the toll in 2007. Over a four-year period, 283 Americans were reported murdered, according to State Department figures.

In the same lapse, more than 35,000 Mexicans have been killed, including about 15,000 last year. The Mexican government says most were gangsters. But hundreds of innocent civilians also have been killed.

"Bystanders, including U.S. citizens, have been injured or killed in violent incidents in various parts of the country, especially, but not exclusively in the northern border region, demonstrating the heightened risk of violence throughout Mexico," the latest State Department travel warning observes.

The warning notes that most of the country, including major beach resorts, remains safe.

"There is no evidence that U.S. tourists have been targeted by criminal elements due to their citizenship," advises the travel warning, which was issued last week. "Nonetheless, while in Mexico you should be aware of your surroundings at all times and exercise particular caution in unfamiliar areas."

Victims of underworld

Many residents along the border have dual U.S.-Mexico citizenship. Some of the murdered Americans may have spent most of their lives in Mexico. Other American border residents frequently cross south of the line to visit friends and family in troubled Mexican towns and cities.

Better than half of the 2010 U.S. victims were killed in Juarez and in Tijuana, which borders San Diego. Both cities are tumultuous binational communities that have become primary underworld battlegrounds.

Among the Americans slain in Juarez last year were Lesley Enriquez, a civilian employee at the U.S. Consulate there, and her husband Arthur Redelfs, an employee of the El Paso County jail. U.S. investigators have arrested members of the Aztecas, a transborder gang that works with the Mexican criminal organizations, in the killings.

In early November, U.S.-born Eder Andres Diaz, 23, and naturalized American Manuel Acosta, 25, both students at the University of Texas at El Paso, were gunned down in Ciudad Juarez. Both were living in Juarez while attending the university.

Not counted in the tally is David Hartley, a 29-year-old oil company employee who disappeared in September after reportedly being attacked by gunmen as he and his wife jet-skied in Mexican waters of Lake Falcon.

His wife said she saw him fatally shot in the head, but Hartley's body has never been recovered. Then again, neither have the bodies of perhaps several thousand Mexicans who have simply disappeared in the violence.

While counseling caution on those traveling in much of Mexico, the U.S. government's warning strongly urges against non-essential travel to Tamaulipas, the state that borders Texas from Laredo to the Gulf Coast.

The warning also emphasizes that Monterrey, Mexico's third largest city, has become risky as well for "local and expatriate communities."

"Local law enforcement has provided little to no response," the warning notes of Monterrey's violence. "In addition, police have been implicated in some of these incidents."

The American toll so far this year includes Brownsville native Jaime Zapata, an agent with the U.S. Immigration and Customs Enforcement who was killed in a February ambush on the busy highway connecting Mexico City to the south Texas border.

Body count rises

A dozen alleged members of the Zetas have been arrested in Zapata's killing.

Zapata was slain little more than two weeks after South Texas-based Christian missionary Nancy Davis, 59, was fatally shot by suspected gangsters near San Fernando, a Tamaulipas farm town 80 miles south of the border at Brownsville.

The town of San Fernando has been a well-identified center of terror since August, when 72 mostly Central American migrants were slaughtered at a rural warehouse outside the town.

Despite government vows to pacify the region following that massacre, the gangsters retained control of it. In recent months, the thugs reportedly have kidnapped and murdered highway travelers and others, burying their remains in a farm village.

So far, 183 bodies have been pulled from clandestine graves near San Fernando this month as officials investigate a long running gangster operation that included pulling travelers from buses.

Chronicle reporter Lise Olsen contributed to this report.

dudley.althaus@chron.com