Ciudad Juarez, the sprawling Mexican metropolis of 1.3 million people across the border from El Paso, Texas, is Murder City, probably the most dangerous city in the world outside a declared war zone.

Already this year, 686 people have been murdered here. Residents hunker in trepidation. Most answer cell phone calls only from people they know to avoid random extortion attempts. Instead of going out on the town, they hold private parties — and only with close friends.

Those residents who can afford to leave have left.

"The exodus is dramatic," said Gustavo de la Rosa, the local ombudsman for the Chihuahua State human rights commission. "There are at least 20,000 abandoned houses, and maybe up to 30,000."

Americans have reason to be concerned, too. The U.S. does about $1 billion a day of trade with Mexico, and nearly one-sixth of that trade goes through the Juarez-El Paso region.

Crime in Juarez also threatens to bleed across the border. Criminal gangs working for drug cartels already operate on both sides of the border, and in a sign of the growing risks, on March 13 gunmen killed three people linked to the U.S. consulate in Juarez. The sky-high murder rate is driven by two rival groups — the Juarez cartel and the Sinaloa cartel — and their battle for control of drug smuggling into the U.S.

The FBI estimates that 40 to 60 percent of the narcotics and marijuana smuggled from Mexico to the U.S. moves through the corridor, which runs roughly from the Texas border with New Mexico to Big Bend National Park, about 300 miles southeast.

Murder is only one of Juarez's problems. Ambitious cartel underlings have diversified into extortion, kidnapping, carjacking and robbery. When President Felipe Calderon sent 10,000 soldiers to Juarez in March 2008 to bolster security after a purge of corrupt police, the army largely ignored other crimes to focus on the cartels, and crime has taken off.

The result is a palpable sense of unease, despite insistence from the mayor, Jose Reyes Ferriz, that only 200 of the 2,400 people killed last year were innocent bystanders.

"The perception of security — that 'it won't affect me' — is less and less," said Carlos Chavira Rodriguez, local head of the Mexican Employers Federation. "Everyone knows you might get robbed in the street or hit by a stray bullet."

Gunmen carried out 1,900 carjackings last year in the city, and extortion is rampant. In a working class district of southeastern Juarez, neighbor Anastasio Sayas surveyed a two-story house that had been torched before dawn. The owners had refused payment to gangsters.

"They demand payment from every small business . . . . It's sad to see how the people suffer," Sayas said.

The 345 assembly plants that ring the city still employ 190,000 workers — making Juarez home to about one-sixth of the assembly jobs in Mexico — and the plants' owners have been told they have nothing to fear from the drug battle.

"The message keeps coming back to these industrial companies that if you're not in the drug trade, you're not going to be a target," said Bob Cook of the El Paso Regional Economic Development Corp.

It's a different matter, however, for tourist-dependent Mexican businesses in the city.

Signs saying "For Rent" in Spanish and English dot shuttered restaurants, bars, hotels and other businesses.

"Six thousand businesses have closed during the last nine months," said Daniel Murguia Lardizabal, head of the local branch of the National Chamber of Commerce. "Downtown is dead."

The miserable employment situation has fueled the ranks of the gangs.

"Juarez has 70,000 young people between the ages of 15 and 29 who neither study nor have jobs," said Chavira, of the Mexican Employers Association. "These people are easy pickings for the gangs."

Those gangs are locked in a fight to the death that experts say threatens anyone who has the misfortune to be nearby.

"To kill one person, they shoot hundreds of rounds and maybe hit the person with five," said Enrique Torres, spokesman for the joint police-military command in the city. "We've arrested people who were paid only 300 pesos ($25) per killing."

The battle for Ciudad Juarez began about two years ago when the Sinaloa drug cartel, led by Joaquin "El Chapo" Guzman and based along Mexico's Pacific coast, began trying to wrest control of the crucial drug smuggling corridor into the U.S. from the Juarez cartel.

Fighting for the Juarez cartel is a street gang known as the Aztecas that operates on both sides of the border. Most Azteca members are heavily tattooed ex-cons who served time in Texas jails. One of the top Azteca leaders, Eduardo Ravelo, is a U.S. citizen.

The Sinaloa cartel's street gangs include the Assassin Artists and the Mejicles, which are less disciplined than the paramilitary-style Aztecas but every bit as homicidal.

Neither side holds the advantage, and the violence is likely to go on.

"Our intelligence does not indicate that the Sinaloa cartel has taken over the Juarez corridor, however they are making serious attempts to do so," said Joseph M. Arabit, the special agent in charge of the El Paso field division of the Drug Enforcement Administration. "The upper hand changes from week to week because this is an ongoing struggle."

Calderon's decision to send in the army in 2007 has seemed only to make matters worse — the death toll of 1,623 in 2008 rose to 2,635 in 2009 — and many residents want the army gone.

Last week, authorities pulled soldiers from the city and turned security over to some 5,000 additional federal police brought to Juarez from around the country.

The city, meanwhile, tries to cope with the executions that average seven per day — so common that this reporter drove by one shortly after it happened. A body lay covered by a sheet on a sidewalk.

On another day, after a hit squad in the Erendira district executed a man, a radio report said children in an adjacent park "kept on playing," paying no heed to the slaying.