And as fresh violence has swept the nation's police force into the center of the drug war – with the unprecedented slaying of at least four high-ranking officers this month – new questions about how many officers are colluding with drug dealers and how effective police efforts are have battered its reputation once more.
At least one slaying was allegedly coordinated by a federal police officer working for drug traffickers. Many local police have resigned. Some have reportedly even sought asylum in the United States.
But Mr. Nuñez is determined to do his part to rebuild confidence in an institution that ranks among society's least trusted. On a recent day, in a hardscrabble neighborhood of Mexico City, he is one of 142 students in a yearlong program at a police academy called the Professional Training Institute (PTI) established to raise the standards of the city's judicial police.
"I feel proud to put on this shirt and feel part of this institution," Nuñez says, before heading out to join his colleagues in calisthenics.
The challenge to stem Mexico's drug violence – which analysts increasingly compare to the situation in Colombia – escalated this month when the acting national police chief was killed at his home in Mexico City. President Felipe Calderón has vowed to fight on, but many say no amount of strength will solve the problem if corruption is not first confronted.
As questions about his strategy surface, especially as the US considers a $1.4 billion aid package for Mexican security forces over three years, smaller grass-roots efforts, such as Mexico City's academy and various nongovernmental organizations around the country, are developing accountability standards and corruption controls that many say could help advance the government's efforts.
"If the police were better trained, it would help against infiltration of corruption in the system," says Jose Arturo Yañez, a professor at the academy. "At the federal level, the training doesn't give them time to develop ethics and the spirit of group work learned in the classroom."
Mr. Calderón has made tackling drug violence the cornerstone of his presidency, sending 25,000 soldiers and police across the country. But at least 1,300 have been killed this year, as wars between drug gangs and security forces rage. Usually low paid, cops often moonlight for drug traffickers.
The killing this month of Edgar Millan, the acting national police chief, represents a troubling new challenge because a federal policeman was arrested for coordinating the job, with alleged plans to target more officers.
"The assassination ... was possible thanks to corruption in the police structure; this is the main weakness of Calderón's general strategy," says Erubiel Tirado, director of the national security program at the Iberoamerican University in Mexico City. "You cannot fight a war like this without a very clean and professional structure."
While it is the federal police who directly investigate drug trafficking, local police are often most susceptible to the lure of drug money.
That is exactly what the PTI is trying to avoid: On a recent day, Nuñez studies the Spanish police model with his colleagues, listening eagerly to the main lesson of the day. "It is about the satisfaction of having fulfilled a duty," agent Mauricio Bautista Lara, their teacher, explains. "Not to get thanks, or more money, or a day off."
Since the one-year program began five years ago, 600 detectives have gone through training, which includes a rigorous recruitment process. Cadets' houses are visited – to make sure their family and friends are not involved in crime or corruption. The point: to begin with a higher caliber of candidate.
The academy is not the only group looking at higher police standards, says Raúl Benitez, a security expert in Mexico City. He says that non-governmental groups have arisen in recent years to demand greater accountability from police at all levels. He is the president of the Collective of Security Analysis with Democracy, formed in 2007. Another group, Insyde, is creating accountability systems for police institutions across the country.
In addition to his military deployment, Calderón pledged to overhaul Mexico's police system. Many support his efforts, in particular reassigning about 300 high- and mid-level officers last year and setting up a new national training academy. "He is trying to make police service attractive by turning it into a career," says Jorge Chabat, a security specialist in Mexico City.
Calderón, and many others, urge patience for the enormous task. "Calderón has made a priority of this transformation," says Armand Peschard-Sverdrup, an expert on Mexico and head of the Washington-based consulting firm Peschard-Sverdrup & Associates, "but it is not going to happen overnight."
Still, others worry that his focus has thus far been one of muscle and not modification. Mr. Tirado says the administration has not been training officers thoroughly because, with the magnitude of the crisis, the effort requires as many officers as possible. Yet the police, who are increasingly met with AK-47s, need more training than ever, he says.
Police deaths are often met with ambivalence – in part because no one knows whether they were targets for colluding with drug cartels or carrying out their missions. A lack of investigation does not help. Mr. Yañez says that, of 300 officers killed over the past two years, only one suspect has been arrested directly for the crime.
This lack of accountability can lower morale: This past weekend, the police chief in Ciudad Juarez, home of much of the nation's worst fighting, resigned from his post. His force had received death threats from drug traffickers, many of which have been carried out.
Demand for change is rising, says Mr. Benitez, and he says places like the PTI are slowly making a difference. "This is an important change in transforming the police. The most important thing is creating a new mentality to serve the citizens," says Mr. Bautista Lara, who spent 12 years as a police detective in Mexico City.
That sense of purpose, for now, seems to be filtering down. Many young recruits hope to be able to continue that spirit. Says Nuñez: "I think we can do important things for our city, and our country."