Nobody wanted the job of police chief in Nuevo Laredo, a city on the U.S.-Mexico border plagued by drug gangs and violence. Finally, Alejandro Dominguez, 52, a businessman and father of three, volunteered to take the post to help his besieged city. Last Wednesday, hours after being sworn in, Dominguez was assassinated by men firing assault rifles from a convoy of Chevrolet Suburbans.

This week, federal troops and police took over the city of 300,000 as the death toll reached 50 in an escalating drug war, and U.S. Ambassador Antonio O. Garza Jr. warned of a "rapidly degenerating situation along the border." The entire local police force was ordered off the streets after city officers engaged in a gun battle with federal investigative police Saturday that left one federal officer seriously wounded.

The human cost of Mexico's aggressive war on drug trafficking is skyrocketing as the country suffers through the worst barrage of drug-related violence in years. More than 600 people have been killed this year, often in remarkably bold and bloody executions, according to national press tallies and state-by-state crime reports.

Nuevo Laredo is just one hot spot in a grisly conflict that has spread across the country. In recent months, a farmer in the Pacific coast state of Sinaloa was gunned down as he visited his parents' grave, and a father in Monterrey, in northern Mexico, was shot dead in front of his son in a video arcade. Authorities have found corpses with limbs chopped off and drums of acid they believe traffickers used to dissolve the bodies of their victims.

"The old code of killing in private -- settling underworld scores in private -- is off," said Ana Maria Salazar, a former drug policy official with the Pentagon who is now a political analyst in Mexico. "Now they are killing in the streets, shopping centers and restaurants."

While many victims of drug violence appear to have been involved in the illicit trade, such as members of rival gangs killed in shootouts, an increasing number are public servants who, like Dominguez, stood up to organized crime, government officials said.

Mexican authorities this week disclosed for the first time that 90 soldiers had been killed in drug-related violence since President Vicente Fox took office in December 2000, vowing a "war without mercy" on Mexico's drug cartels. In addition, at least 65 agents of the Federal Investigative Agency have been killed since it was formed in 2002.

Some died in confrontations with drug dealers on city streets; others crashed in helicopters after traffickers shot them down or disabled them by stringing heavy cables across narrow valleys where opium poppies and marijuana grow.

In a recent interview, Fox likened Mexico's "explosion of organized crime killing" to the Al Capone era in Chicago. "Let's recall Chicago in the early '20s. I mean it took years to get rid of the mafias, it took years to get rid of organized crime," he said. Fox said U.S. and Mexican authorities were working jointly to confront criminals who control "millions and millions and millions of dollars."

Federal officials said that frequently their enemies are not just the drug cartels, but local police who have been corrupted by drug money. That problem has unleashed a growing battle between federal police officers, many of them trained by U.S. law enforcement, and their local counterparts, underscored by the recent clash in Nuevo Laredo.

"It's evident that there are police forces that have been penetrated by organized crime," presidential spokesman Ruben Aguilar Valenzuela said Monday.

Over the past decade, numerous federal and state police officers, as well as soldiers, have also been implicated in drug trafficking.

Aguilar announced that hundreds of soldiers and federal agents had been ordered into areas racked by drug violence. He also called on U.S. authorities to help stop the flow of weapons into Mexico. He said much of the high-powered weaponry and other technology used by the drug cartels is smuggled in from the United States.

Law enforcement officials trace the current violence to the Fox administration's arrests of 15 leaders of billion-dollar criminal organizations. Rivals have tried to move in and violently take over their territory. There have also been bloody battles within cartels, as lieutenants fight to move up the ladder because of new vacancies at the top, Mexican officials said.

Officials in Washington have continually praised the efforts of Fox, noting he has jailed more top cartel leaders than any Mexican president in history. Since he took office, the Mexican government has arrested more than 46,000 people on drug charges, destroyed numerous clandestine landing strips and wiped out thousands of acres of poppy and marijuana fields.

The aggressive new efforts to combat drug traffic by elite federal police teams have, however, often ended in tragedy. Last month, for example, a federal police officer chasing a bulletproof Ford Expedition in the border city of Matamoros was shot and killed by the occupants, who escaped on foot. The vehicle was found to contain four grenades, five AR-15 rifles, three MP5 submachine guns, two telescopes, 11 cell phones and more than 2,000 rounds of ammunition.

In the past, Mexican and U.S. officials said, law enforcement officers might not have given chase, either out of fear or because they were on the drug cartels' payroll. Officials believe Ezequiel "Tony Tormenta" Cardenas, the brother of jailed cartel boss Osiel Cardenas Guillen, was a passenger in the SUV.

"The war goes on, and we're on the good side," said Carlos Diaz, 22, the partner of the slain officer. "Maybe not everyone is brave enough to come and work here, but many people do, and they are very brave. You know the danger. You know that when you leave your house, you might not come back."

A number of analysts have begun pointing out that Fox's much-applauded effort against drug traffickers has not reduced the supply of drugs reaching U.S. streets from Mexico. Instead, they said, all Mexico has gotten for its effort is more violence and a rapidly rising drug consumption problem.

"The good news is that there are more capos in jail; the bad news is that it doesn't change anything," said Jorge Chabat, an academic who studies justice issues. "There's no change in the amount of drugs available on the street, and you have more violence. The logical question is, 'What are we doing this for?' "

Jose Luis Santiago Vasconcelos, the nation's top organized crime prosecutor, said in an interview that it was important to prevent Mexico from becoming like Colombia, where drug traffickers in the 1980s took over much of the country by bribing, intimidating or killing politicians, police officers, judges and prosecutors. If Mexico does not attack the drug traffickers, he said, "our children are going to be suffering tomorrow, with big drug traffickers who try to dominate . . . our institutions, to run the justice system, the media, even our lives."

Currently, Mexico's most notorious drug baron is Joaquin "El Chapo" Guzman, who escaped from a maximum-security prison in January 2001, a month after Fox took office. Salazar noted that the country's weak prison system has not been able to contain the leaders, several of whom have escaped, while others who remain confined have recently been found to be ordering cocaine shipments and assassinations by cell phones.

"They can't control the capos in jail, so how can they do it on the streets?" Salazar said.

Guzman is believed to be hiding in the mountains of Sinaloa, his home state. The U.S. government has posted a $5 million reward for his arrest. Officials said local police and residents have been sheltering Guzman and tipping him off whenever law enforcement officials show up.

Vasconcelos lamented that Guzman, who he said freely spreads cash around in poor communities, is regarded as a hero by many in his home state. The prosecutor implored the Mexican media to help "change that image."

"Help me make the people realize that this peso or this dollar that the drug trafficker gives is dripping in blood," he said.

The entire Nuevo Laredo police force was ordered off the streets after a shootout with federal police.Alejandro Dominguez was assassinated hours after he was sworn in as Nuevo Laredo police chief.