( CNN ) -- They were elected to serve their small towns , but recently , mayors throughout Mexico have found themselves the targets of violent killings believed to be perpetrated by drug cartels . As the cartels face challenges from the military and rival cartels , many of the groups have sought to increase their influence over the towns in the areas where they operate . Not only do they traffic in drugs , but cartels have diversified into human smuggling , extortion , and even into public works . A typical drug trafficking organization has a network of lookouts on city streets , `` tax '' collectors who collect protection money , and hit men who enforce their will on populations . The result has been situations where the duties of mayors -- to collect taxes , provide security , and carry out public works -- have butted against the interests of the cartels , oftentimes with fatal results . Thirteen mayors have been killed so far this year in Mexico , in states around the country . The killings represent just a fraction of all the mayors throughout the country , but illustrate a disturbing new development in the drug war . The turf wars that the cartels are fighting are reflected in the office of the mayor , said Ana Maria Salazar , a television and radio political commentator in Mexico City . `` These groups are demanding more from the mayors , '' Salazar told CNN . The cartels seek information , police protection and means of communication from the local governments , she said , adding that she was surprised that more mayors have not been killed . This year , the largest number of mayors -- four -- have been killed in the border state of Chihuahua , a hotbed of fighting between the Juarez cartel and the Sinaloa cartel . Killings have also been recorded in hot spots such as the state of Michoacan , home to the Familia Michoacana cartel . Also affected have been Tamaulipas and Nuevo Leon , two border states where the Zetas drug cartel is battling an alliance comprised of the Sinaloans , La Familia Michoacana and the Gulf cartel . George Grayson , a Mexico expert and professor of government at the College of William and Mary , said that he sees the Zetas , who got their start as army defectors , as a force behind some of the killings . The Zetas have been on the defensive against the alliance that formed against them , as well as from the government forces . `` They are trying to demonstrate they are still important players , '' Grayson said . `` They want to regain the reputation they think they have lost as the most ruthless organization in the Americas . '' While the Zetas are not known for doing public works , some of their illicit activities -- collecting money for protection , for example -- shadow the responsibilities of the local government . `` You are expected at least to turn a blind eye , if you are a mayor , especially , '' Grayson told CNN . Most of the violence against mayors has happened in small towns in drug cartel territory , where the leaders are more vulnerable than their big-city counterparts . One example is the small border town of Puerto Palomas , known locally simply as Palomas , in Chihuahua . A year ago this month , the mayor , Estanislao Garcia , was kidnapped and then found executed . Palomas , located across the border from Columbus , New Mexico , literally scrapes along the border with the United States . As a town , it is known as a staging area for immigrants looking to steal across the border , and lacks an industry of its own . `` It 's been a difficult challenge , '' Maria Lopez , the current mayor of Palomas , said of her position . But since that killing , things have been relatively calm . `` I have not had any problems , thank God , '' she told CNN . At first , she says that she was not scared when she succeeded Garcia . Then , she restated her answer . `` I first I was scared , why should I lie ? But not anymore '' No drug traffickers have come to negotiate with her on any issues , she said . Lopez says she does n't know who killed Garcia , though speculation is that it was one of the cartels that work in the area . `` It 's sad . It 's truly sad . I do n't know what is behind it , '' she said . Garcia had been in a quarrel with local teachers at the time of his death , but she does n't think they were the ones who ended his life . Other killings offer more clear evidence of cartel involvement . In September , in the town of El Naranjo in San Luis Potosi , witnesses saw four armed and hooded men exit a white truck in front of city hall in the early afternoon . Two men waited in front of the building while the other two went inside Mayor Alexander Lopez 's office and shot him . Other killings have been even more violent . The mayor of Tancitaro , Michoacan -- Gustavo Sanchez -- was found stoned to death on the side of the road . An aide who was with him met the same fate . Being a mayor in Mexico these days requires great courage because they face intimidation , threats and bribes , Mayor Jaime Dominguez of Ascension , Chihuahua , told CNN . `` As mayors we are under a lot of risk , '' he said . Dominguez says that he tries to focus on his job , helping the people of his community with infrastructure and water projects in his agricultural town . The only crime he is concerned with are local ones , such as robbery and domestic violence . Drug trafficking and murders fall to the state and federal authorities to investigate . `` Our job is to work closely with the community , '' he said . `` I feel safe , I feel secure . My hands are clean . '' Still , he wishes for more federal and state help for issues such as social development and security . `` In a way , this ( violence ) was expected , '' Salazar said , referring to how cartels continue to push the limit to who they can kill . Before the mayors were targeted , cartels had been targeting police chiefs throughout the country . Grayson agreed . `` I think the death toll will rise with the mayors . '' Could governors be next ? A leading gubernatorial candidate in Tamaulipas state , Rodolfo Torre Cantu , was gunned down in June .
Thirteen mayors have been killed in Mexico this year . Most are from small towns where they are more vulnerable . Comment: The violence against them could increase , analysts say .