The mayor of Chilpancingo, the capital of Mexico's violence-plagued state of Guerrero, travelled alone to an out-of-town meeting before he was killed, Mexico's security minister said on Tuesday about a murder which has shocked the country.

Alejandro Arcos, of Mexico's main opposition alliance, had only taken office six days earlier. Photos circulated on Sunday via WhatsApp and in Mexican media outlets depicting a severed head on top of a pick-up truck, appearing to be that of Arcos.

The gruesome act happened just days before new President Claudia Sheinbaum unveiled her security strategy aimed at combating violent crime.

Security Minister Omar Garcia Harfuch said in a news conference that an investigation had determined Arcos left Chilpancingo in his pickup truck without a driver or escort heading toward the nearby town of Petaquillas.

Arcos had not requested security protection from the federal government, Garcia Harfuch said.

"We know that he was going to a specific meeting, that he was not accompanied by someone. Communication was lost in a community, and... hours later the mayor's body was found," Garcia Harfuch said.

Asked by a reporter whom Arcos was meeting with, the minister declined to give details, saying it was an open investigation.

"There is a lot of information on this subject that we must guard for the sake of the investigation," Harfuch said.

Mexican newspaper Reforma, citing federal government sources, reported that Arcos had met with members of Los Ardillos, a criminal group active in Guerrero state, hours before his death.

A funeral for Arcos was held on Monday, with mourners lining the streets and throwing rose petals as his casket was driven through the town.

Similar grisly murders involving decapitations have plagued Guerrero - including Chilpancingo - in the past as a way cartel members use to send messages of threats and control.

As recently as 2023, violence from warring gang factions paralyzed public transportation and shuttered businesses in Chilpancingo. After Arcos' killing, classes were suspended for local schools.