GUEST: David Shirk, associate professor of political science, University of San Diego Transcript for audioclip 29140

Homicides are up in Mexico after years of decline that started when President Enrique Peña Nieto took office in 2012, according to a new report by researchers at the University of San Diego.

The report, titled "Drug Violence in Mexico," shows homicides increased by 8 percent from 2014 to 2015. But researchers say it's too early to tell if this signals a reversal of the downward trend.

"For one thing, it suggests that previous progress in reducing homicides was not necessarily attributable to the increased effectiveness of law enforcement or government policy," the report states. "For another thing, it raises questions about current dynamics among organized crime groups, and whether there is potential for escalation moving forward."

In Tijuana, while homicides are down from the record 2008 levels, homicides jumped from 462 in 2014 to 612 in 2015. Tijuana remains the second most violent city in Mexico after Acapulco.

Researchers say the reasons behind the recent increase in homicides are complex, and range they from economics to legal accountability. But one contributing factor could be Mexico's shifting drug trafficking landscape and the emergence of "cartelitos."

"The landscape of drug trafficking in Mexico now appears to be dominated by one powerful 'cartel' amid many cartelitos," the researchers say. "Because most of the smaller, regional criminal organizations have far lesser capability to finance and manage major drug trafficking operations, these cartelitos are arguably a much greater threat to public security, in that they obtain revenue through kidnapping, robbery, and extortion."

David Shirk, associate political science professor at the University of San Diego and co-author of the report, discusses the findings on KPBS Midday Edition Midday.

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