When public safety chief Genaro Garcia Luna found out, he canned the team, all 50 to 60 members. "He fired everybody," a senior U.S. law enforcement official said.

But the episode shows how difficult it will be for Mexico to reverse a legacy of police corruption that has tainted whole departments, shattered people's faith in law enforcement and compromised one of society's most basic institutions.

President Felipe Calderon's 3-year-old drug offensive has laid bare the extent to which crime syndicates have infiltrated police agencies at virtually every level. By blurring the line between crime fighters and gangsters, the rampant graft stands as one of the biggest impediments to the Calderon campaign.

Amid the raging drug war, Mexican officials are trying to fix the police through a hurried nationwide effort that includes better screening and training for candidates on a scale never tried here before. At the heart of the overhaul is a "new police model" that stresses technical sophistication and trustworthiness, and that treats police work as a professional career -- not a fallback job.

In steps that are groundbreaking for Mexico, cadets and veteran cops are being forced to bare their credit-card and bank accounts, submit to polygraph tests and even to disclose their family members to screeners to prove that they have no shady connections. Across Mexico, hundreds of state and municipal officers have been purged from their departments, and scores more arrested on charges of colluding with drug gangs.

But Mexico has a habit of trading in one corrupt police agency for another, and it will be a long, uphill struggle to create a law enforcement system that can confront crime and gain the trust of ordinary Mexicans. Until then, crooked cops undermine efforts to strengthen rule of law and defeat drug cartels.

"If you don't have a safe environment to conduct investigations, then it's going to be extremely difficult to capture the narcos," said the U.S. law enforcement official, who was not authorized to speak publicly. "If you have state police that are corrupt and constantly feeding your movements, investigative movements, to the bad guys, you're not going to get anywhere."

Some people fear that the soaring drug violence and mistrust toward police could spark the formation of death squads or vigilante groups. There have been suspicions, although no proof, that dozens of killings have been committed by people taking the law into their own hands. More than 13,800 people have been slain since Mr. Calderon declared war on the drug cartels, according to unofficial news media tallies.

Although Mexican federal police are in charge of the crackdown against the cartels, it is at the state and municipal levels where law enforcement is most vulnerable, officials and analysts say. Drug gangs exploit hometown ties, dangle bribes and threaten the lives of officers and their relatives to turn police into a kind of fifth column. Poorly paid state and municipal officers are often on the payroll of drug smugglers, passing tips, providing muscle or looking the other way when illegal drugs are shipped through their turf.

Criminal infiltration of local departments has worsened as the Mexican political system becomes less centralized, and as narcotics traffickers delve into offshoot enterprises -- such as kidnapping, theft and extortion -- that under Mexican law fall within the jurisdiction of state authorities.

At times, local police have faced off in tense showdowns against Mexican federal police and soldiers. The mistrust often prompts federal authorities to keep their state and municipal counterparts in the dark, aggravating interagency frictions.

"There is a disorganized police fighting against organized crime," said Guillermo Zepeda, a police expert at the Center of Research for Development in Mexico City.

In the western state of Michoacan, 10 municipal officers were arrested for the slayings of 12 federal agents there in July. In the Gulf of Mexico port city of Veracruz, authorities investigating the June disappearance of customs administrator Francisco Serrano detained nearly 50 municipal officers. The chief of municipal police for the seaport and three traffic officers were later charged with kidnapping him. Mr. Serrano has not surfaced.

The profound flaws of Mexico's police, who frequently are ill trained, poorly equipped and unhappy in their work, are the most visible emblems of how the drug offensive is straining the nation's broader system of law and order.

An opaque and creaky court system groans under the weight of thousands of new drug-war cases, and a number of prosecutors, defense lawyers and judges have been slain. Meanwhile, prison officials scramble to make room for the surge of detainees, many of them violent.

Mr. Calderon's administration has laid out a strategy to expand and revamp the federal police and to force states, cities and towns to modernize and clean up their forces with tools such as polygraphs and drug tests. Standing in the way are many years of graft, turf jealousies, budget constraints and a drug underworld that has greeted every government move with greater viciousness.

Mr. Garcia Luna, the public safety chief, has seized the moment to hire thousands of federal cadets, who under the strict new standards must hold at least a university degree. Despite the stiff requirements, the federal force has grown to 32,264 officers, from around 25,000 a year ago.

The U.S. government supports the push to expand and professionalize Mexico's federal forces, lending dozens of police instructors as part of a $1.4 billion aid package for Mexico, known as the Merida Initiative.