Serna, a member of the ruling Institutional Revolutionary Party, was one of at least five politicians killed in the past week in Mexico on the eve of an important election year.
Two days earlier, the mayor of another Guerrero town, Petatlan, about two hours north along the coast, was killed while eating with friends at a restaurant. And the day before that, a state congressman from Jalisco was gunned down while driving with his son. A former state congressional candidate and a town council member also were killed in the past week.
The violence was another reminder of the serious dangers inherent in Mexican politics, particularly at the local level, where drug gangs regularly exert influence. It also has prompted politicians from different parties to call for tighter security and to demand justice ahead of elections for more than 3,400 positions at all levels, including the presidency, this summer.
Four of the five politicians killed were affiliated with the leftist Party of the Democratic Revolution (PRD). Top party officials have condemned the violence and asked to meet with federal officials to discuss the cases.
"We are six months from the presidential election, and of course these attacks against our members are taken as a warning against participating," Ángel Ávila Romero, secretary general of the PRD, said last week, according to El Universal newspaper.
The killing of politicians has been a recurring problem in Mexico in recent years. Since President Enrique Peña Nieto's administration began in 2012, 61 current or former mayors have been slain, up from 49 killed during the previous administration, according to a count by the National Association of Mayors.
"We have called on the president asking for an immediate meeting to implement a security protocol for mayors," Enrique Vargas del Villar, the president of the association, said in a phone interview. "The insecurity cannot continue this way in our country."
Vargas del Villar said mayors needed direct phone lines to the Interior Ministry to report threats against them so federal officials could intervene.
In states where drug gangs and cartels maintain a strong presence, some officials face extortion demands or other threats. Others have developed ties to these criminal groups, according to security experts.
"This shows the breakdown of institutions due to the penetration of organized-crime groups that apparently try to influence the electoral process," said Miguel Arroyo Ramírez, a lawyer and founding member of an anti-crime civil society group. "When someone appears who doesn't share their interests or has different interests, these groups don't have the slightest hesitation in eliminating those who are inconvenient."
The motives behind this past week's political killings remain unclear.
The mayor of Petatlan, Arturo Gómez Pérez, was shot at point-blank range Friday inside a restaurant in his town in front of several witnesses, according to local news reports. The governor of Guerrero, Héctor Astudillo, called the shooting a "cowardly and cunning crime" against a man "who behaved correctly and with great commitment to his people."
"We reject and repudiate this act," he wrote on Twitter.
On Thursday, Saúl Galindo, a PRD state congressman from Jalisco and president of the justice committee, was shot while driving near his ranch in Tomatlan, authorities said. He had served as mayor of Tomatlan and reportedly was planning to run for the position again.
On Saturday, Gabriel Hernández Arias, a town council member in Jalapa in the state of Tabasco, was found stabbed to death in his home, according to authorities.
The same day, lawyer Juan José Castro Crespo was killed in Mexicali, a city in the border state of Baja California. Castro Crespo had been a PRD candidate for state congress and president of the local bar association.
"In addition to being dismayed, the political class needs to step up its security measures," Heriberto Huicochea Vázquez, the head of the Institutional Revolutionary Party in Guerrero, told the news site Bajo Palabra. He called on authorities to conduct rigorous investigations "so we can at least know the motive for why these homicides have occurred."
Gabriela Martinez contributed to this report.