Police on Mexico’s Caribbean coast said Friday they have found the bodies of four men, including a policeman, shot to death near a resort just south of Cancun.

The victims all had apparently been shot in the head and were found dumped near the highway leading out of Cancun, said Luis Rodríguez Bucio, the assistant head of Mexico’s Public Safety Department.

A former Cancun police officer was found at the scene going through the pockets of one of the bodies, and said the dead man was a friend. He was detained by officers, police said.

The four victims included a Cancun police officer who had been accused of accepting bribes to protect brothels, but who had been kept on the force anyway. He was on medical leave at the time of his death.

Authorities said the bodies were found Thursday, and said the victims may have been involved in providing protection for migrant smugglers. Bucio said the killings might also be related to a raid on a property in Cancun earlier this year in which guns and drugs were seized.

Drug cartels in Mexico constantly kill rivals to protect their territories or businesses. Officials said Friday that over two-thirds of the 88 murders that took place Thursday nationwide in Mexico were believed to be related to drug cartels.

The Caribbean coast, and especially Cancun, is often used by smugglers to illegally move migrants through Mexico, in part because the resort has good airline connections and there are so many foreigners that migrants can be moved around without drawing as much attention.

Drug cartels in Mexico have increasingly become involved in migrant smuggling in recent years. The cartels also fight for control of the drug trade in the resorts along the coast.

The deaths come just days after a authorities said a 12-year-old boy was killed after gunmen on jet skis opened fire at a beach in Cancun.

Territorial disputes between drug dealers have turned deadly in the resorts along Mexico’s Caribbean coast in recent years. In May, 10 bodies were found scattered in the country’s once-glamorous resort city of Acapulco, which has been engulfed by violence linked to cartels, local security officials said.

Earlier this year, the U.S. warned American citizens traveling to Mexico for spring break to exercise increased caution.  The advisory said travelers should be particularly cautious in downtown areas of popular destinations, including Cancun, Playa del Carmen and Tulum.