The charred remains of three federal detectives have been found in the bed of a burning pickup truck in the southern Mexico resort of Zihuatanejo.

A fourth person's remains were found nearby Friday along with a hand-lettered sign containing an apparent reference to a drug gang.

The security spokesman for the southern state of Guerrero said the detectives' bodies had been covered with logs and set afire. Their weapons were also burned.

Their vehicle was found on the outskirts of Zihuatanejo in an area not frequented by tourists.

Spokesman Roberto Alvarez said the agents were reported missing on Thursday.

The area has been the scene of increasing violence in recent months.

Guerrero state has seen an uptick in killings, with gangs often sending messages to rivals by dumping bodies.