MEXICO CITY — Mexican investigators said Thursday they have found 166 skulls in clandestine mass burial pits in the Gulf coast state of Veracruz.

Veracruz state prosecutor Jorge Winckler said that for security reasons he would not reveal the location of the site.

Winckler said the bodies were buried at least two years ago. He said investigator had found 114 ID cards in the field, which held about 32 burial pits.

It is one of the largest mass graves discovered so far in Mexico, where drug cartels frequently use such clandestine pits to dispose of their victims.

Investigators found clothes, personal possession and other parts of skeletons in the pits, but they focused on the skulls in counting, because each corresponds to one person.

Veracruz was the scene of bloody turf battles between the Zetas and Jalisco drug cartels, but the state also suffered waves of kidnappings and extortions.

Winckler said prosecutors found the field after a witness told them that “hundreds of bodies” were buried there.

Investigators used drones, probes and ground-penetrating radar to locate the pits and began digging about a month ago.

Winckler did not rule out finding more bodies.

It was not the first time that someone with inside knowledge of mass graves revealed their location.

In 2016 and 2017, Veracruz investigators found 253 skulls and bodies in burial pits outside the state capital, after relatives of missing people said they received a hand-drawn map from someone detailing the location of the graves.

In 2011, police found 236 bodies in burial pits in the capital of northern Durango state, which is also named Durango.

A total of 193 corpses were found in the town of San Fernando in Tamaulipas state, just north of Veracruz. Officials say most of those were Mexican migrants heading to the United States who were kidnapped off buses and killed by the Zetas cartel.