Opposition politicians and even some police officials have begun to question whether the president’s ambition has exceeded his grasp, with dangerous and destabilizing consequences for a country that shares a 2,000-mile border with the United States. Bush administration officials have said Mr. Calderón’s efforts might founder unless the United States Congress approves a $1.4 billion package of equipment and training over three years for Mexico’s police.

Top security officials who were once thought untouchable have been gunned down in Mexico City, four in the last month alone. Drug dealers killed another seven federal agents this year in retaliation for drug busts in border towns. Others have died in shootouts.

Drug traffickers have killed at least 170 local police officers as well, among them at least a score of municipal police commanders, since Mr. Calderón took office. Some were believed to have been corrupt officers who had sold out to drug gangs and were killed by rival gangsters, investigators say. Others were killed for doing their jobs.

The president has vowed to stay the course, portraying the violence among gangs and attacks on the police as a sign of success rather than failure. The government has smashed the cartels, he says, forcing a war among the splinter groups. The killing of Commissioner Millán, he has said, was “a desperate act to weaken the federal police.”

“What it signifies is a strategy of some criminal organizations who seek to terrorize society and paralyze the government,” he said last week. “The question is, should we persevere and go forward or simply hide in our offices and duck our heads. No way is the Mexican government going to back down in such a fight.”

The violence between drug cartels that Mr. Calderón has sought to end has only worsened over the past year and a half. The death toll has jumped 47 percent to 1,378 this year, prosecutors say. All told, 4,125 people have been killed in drug violence since Mr. Calderón took office.

But the steady drumbeat of police killings has caused more shock here. On Wednesday, for instance, the second in command of the police in Morelos State and his driver were found dead in the trunk of a car. A placard on the bodies warned against joining the Sinaloa Cartel.

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Several terrified local police chiefs have resigned, the most recent being Guillermo Prieto, the chief in Ciudad Juárez, who stepped down last week after his second in command was killed a few days earlier.

“It is not just happening in Ciudad Juárez,” Mayor José Reyes Ferriz, said at the funeral for the deputy commander, Juan Antonio Roman García. “It’s happening in Nuevo Laredo, in Tijuana, in this entire region. They are attacking top commanders to destabilize the police.”

One reason for the surge in violence is that Mr. Calderón and his public security minister, Genaro García Luna, have upset longstanding arrangements between the police and drug traffickers at every level of government, several experts on crime in Mexico said.

Last year, Mr. García Luna removed 284 federal police commanders across the country, replacing them with his own handpicked officers, many from outside the force, who had been trained at a new academy and who had been closely vetted for signs of corruption.

He has also restructured the department, demoting dozens of career officers and putting in command people he trusts — a small circle of highly educated outsiders, most with a background in the military or in Mexico’s espionage service.

Most of these commanders also served under Mr. García Luna in the previous administration of President Vicente Fox as part of the Federal Investigation Agency, or A.F.I., an elite force modeled on the F.B.I.

The agency showed results. President Fox’s government arrested several of the country’s most notorious drug kingpins, among them Osiel Cárdenas, leader of the Gulf Cartel, and Benjamín Arellano Félix, who controlled Tijuana. The arrests caused turmoil inside the cartels and turf wars among them.

When he took office, President Calderón merged the investigative agency with the existing federal police force and put Mr. García Luna in charge. Over the past 18 months, the new force has recruited heavily among college students and former soldiers. The government has raised the starting pay for officers and greatly improved training.

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But even with about 3,000 new recruits, the Calderón administration has yet to purge the force of thousands of career officers with roots in the old force, which was rife with corruption. Many of these officers have dubious loyalties and made money from graft, especially those assigned to highways, ports and airports, according to criminologists and police officials.

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“To train these people and get them out on the streets is going to take at least a couple of years,” said Bruce Bagley, a professor at the University of Miami in Coral Gables, Fla., who has studied drug trafficking throughout Latin America. “That leaves much of the rotten core of the police still in place.”

At the same time, Mr. Calderón and his predecessor have largely dismantled the state security apparatus that kept an iron grip on Mexico for decades when it was ruled by a single party, the Institutional Revolutionary Party. The Intelligence Agency and the Interior Ministry have been stripped of their extensive networks of informants.

As a result, some critics say, the new federal police force not only lacks the intelligence it once had, but is full of disgruntled officers and commanders who have lost their positions or, in some cases, their sources of graft.

One of those officers was José Antonio Martín Montes Garfias, the man whom investigators accuse of hiring an assassin to kill Commissioner Millán. Officer Montes Garfias had long worked at Mexico City’s international airport, one of the main entry points for cocaine and chemicals used to make methamphetamine, and he was suspected of protecting shipments for the Sinaloa Cartel. Commissioner Millán had him transferred.

The police also say Mr. Montes Garfias had a hand in the killing of Roberto Velasco Bravo, the chief of the organized crime division in the public security department, on May 1. When he was arrested, Mr. Montes Garfias had documents from several cars used by other top-ranking federal police officials, showing names, license plates and models.

Prosecutors say corrupt officers also tipped off gunmen who killed Omar Ramírez, a high-ranking A.F.I. commander, last September as he drove on a busy street in downtown Mexico City. Mr. Ramírez had left his office for an urgent meeting at an unusual hour, yet the gunmen knew his route. Prosecutors say he was killed for making too much progress in investigating the Gulf Cartel.

Yet some police commanders say corrupt officers are less of a problem than the lack of information about drug dealers. They also complain that the intelligence arms of the military and the police do not share information until they are on the point of making a raid, for fear of leaks.

Commissioner Javier Herrera Valles oversaw President Calderón’s efforts to restore order in various states for 10 months until he was demoted last February after openly criticizing the operations in a letter to the president.

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Mr. Herrera maintains that the federal police are acting on wisps of information, like tips from anonymous callers. They have very little hard evidence from undercover officers, wiretaps or surveillance. The operations consist mostly of stopping trucks at checkpoints and endless patrols through neighborhoods, he said.

“They don’t have any good intelligence gathering,” he said in an interview. “We were patrolling without any direction. Going in circles, nothing else.”

Mr. Calderón and his top security officials disagree. They point out that the government has made record seizures of cocaine, marijuana and caches of arms over the last year and a half. They have also arrested scores of people alleged to be hired guns for the cartels, along with a handful of high-level drug dealers.

Antonio Guzmán, who commands the 640 federal agents sent in recent weeks to Sinaloa to hunt the gang leaders believed to have been behind Commissioner Millán’s killing, denied that the killings of police officials had dampened his officers’ spirits.

“It hasn’t affected morale,” Mr. Guzmán said as he patrolled the streets at the head of a column of four pickups full of heavily armed officers in black garb with machine guns and flak jackets. “We know what we are getting into here. If anything we have more desire to win, because we cannot permit this to continue.”

He acknowledged the leaders of the Sinaloa gangs were probably long gone, having fled to the mountains or to other states. He said the real reason his force had been sent in was to instill confidence in residents that the government could protect them.

Yet residents said the patrols and checkpoints only helped as long as they were there. Several said that the drug dealers in the neighborhood were well known, but that no one dares name them to the police. “We don’t mess with them,” said Wilfredo Valenzuela, 35, a mechanic.

Alma Rosa Camacho López, 42, the janitor at a local grade school, said that as soon as the federal officers leave, the drug dealers come out of hiding. “We need the government to be on top of these people all the time,” she said. “When they leave the same problem will come back and we will be in the same fix as before.”