Cartel killings of mayors in Mexico have reached record numbers, but the motivations behind them are not as simple as “plata o plomo,” meaning silver or lead — a choice between receiving money or bullets.

Thirty-seven mayors, former mayors, and mayoral candidates were killed in 2018, a record high and two more than the previous year, according to a recent report. Researchers at Rice University analyzed these killings and found that the systematic targeting of local officials began in 2004 and have increased steadily over the next 14 years.

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From July 2004 to March 2018, of the killings with an identifiable cause, half of them were attributed to organized crime. Former mayors were almost as twice as likely to be killed as incumbent ones. Mayors of smaller municipalities were also more vulnerable, with 53 percent of the killings occurring in municipalities with 20,000 residents or less.

The consequences for crossing the cartels also seem to be becoming more immediate. Earlier this year, a mayor in Oaxaca was killed less than two hours after taking office.

The government of President Andrés Manuel López Obrador has announced that a division of the new National Guard will focus on protecting government officials and facilities.

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The motivations behind the killings of mayors are diverse, from the silencing of those who have tried to combat the criminal groups, to retaliation against those with cartel connections, to the pressuring of small town mayors to provide criminal groups free rein and key information.

The mayor of Tiquicheo in Michoacán survived two assassination attempts in 2009 after speaking out against organized crime. In 2012, she was kidnapped in broad daylight by cartel members and her body was later found dumped in a rural community.

The geographic distribution of the assassinations is also important for understanding the evolving criminal dynamics in Mexico. Since President Felipe Calderón initiated his war on drugs in 2006, the large cartels have fragmented.

“This has led to more organizations that are highly violent and highly flexible,” David Pérez Esparza, writer of the Rice University Report and now head of Mexico’s National Information Center, told InSight Crime.

He added that a number of factors have contributed to a shift in cartel strategy that has fueled the targeting of local officials. The legalization of marijuana in many US states and the increased popularity of fentanyl has decreased the profitability of trafficking heroin and cannabis. This has led to a greater focus on locally based criminal economies, such as oil theft, extortion and other criminal activities that require control of smaller amounts of territory, according to Pérez Esparza.

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Mayors serve as sources of knowledge that cartels can mine in pursuit of their illicit activities. They are often deeply knowledgeable about a municipality, including infrastructure projects, natural resources and wealthy businessmen that could be targeted for extortion. They also have details of upcoming federal police and army operations, as well as knowledge of which criminal groups are dominant within an area.

In smaller municipalities mayors are especially vulnerable to being killed for refusing to cooperate with cartels. “In rural areas, mayors are reliant [for their safety] on police that often do not work properly, making them easier to target,” said Pérez Esparza. Their murders also attract less attention than that of mayors in more populous areas. Fearing death, some mayors choose to opt to work alongside the cartels, although this too puts them at risk.

Officials have also been targeted by criminal groups for real or alleged links to rival organizations. Mexico’s mayors have been tied to cartels on multiple occasions. In one notable operation, ten mayors were arrested by federal police for providing information and protection to La Familia Michoacána cartel.

“Public suspicion of mayors after they are killed is high and some citizens assume that their deaths are a result of involvement in organized crime. This diminishes public outcry and calls for thorough investigations,” Helden De Paz Mancera, another writer on the report, told InSight Crime. “For citizens suffering from high levels of violence themselves, there is little incentive to demand protection for politicians that they view as corrupt.”

Finally, the killing of mayors is also a way for the cartels to show their power or send a message. Mayors have been killed mere days or hours after taking office, demonstrating the ability of the cartels to target politicians at will.

In January 2016, cartel hitmen killed Gisela Mota in her house in front of her family mere hours after she was sworn into office. Her killing was likely meant to send a signal to other mayors in the state of Morelos, 13 of whom had already been threatened by Los Rojos, the group responsible for the murder.