Prosecutors in north-central Mexico said Tuesday they have found the severed heads of four men in Styrofoam coolers along with gang messages directed at rivals.

An official of the Zacatecas state prosecutor's office said the heads were found packed in ice on a roadside Monday, along with hand-written signs. He spoke on condition of anonymity because he wasn't authorized to be quoted by name

The victims have not yet been identified and their bodies have not yet been found.

But the official said the content of the messages suggested the victims were members of a drug gang.

"These are the kind of messages that drug cartels leave for other gangs ... as part of their disputes," the official said.

Photos showed some of the coolers bore the letters "CDG", a reference to the Gulf Cartel, and the words "Welcome to Hell."

The area was once dominated by the highly violent Zetas cartel, much of whose leadership has been arrested or killed.

The Gulf and Jalisco New Generation cartels are now believed to be moving into the area.