A secret mass burial site containing at least 166 skulls has been discovered, Mexican officials announced Thursday. The site is one of the largest mass graves found to date in the country.

The bodies — found in the Gulf coast state of Veracruz — had been buried for at least two years, according to a translation of a release from the State of Veracruz's prosecutor's office.

The region has been the site of bloody conflict between drug cartels, and the area has suffered from waves of extortion and kidnapping.

Officials have found clothes, personal possessions and other skeletal remains at the site, but skulls were used for counting purposes. Investigators discovered about 32 burial pits at the site.

State prosecutor Jorge Winckler said that the location of the graves would not be disclosed for security reasons.

Relatives of missing persons will have access to a photographic record of clothes, IDs and accessories found in the graves, a release says.

Hundreds of bodies have been found in burial pits in Veracruz before — 253 skulls were discovered in 2016 and 2017.

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Around 29,000 murders were recorded in Mexico last year, a record high, according to a July Congressional Research Center report. That comes as previously stable Mexican drug cartels have been violently splintering.

Victims often include cartel members and people who are resisting recruitment into cartels, the BBC reports. Relatives of disappeared people have criticized the Mexican government, saying more could be done to help them locate loved ones, according to the publication.

More than 5,000 people are believed to have gone missing between 2010 and 2016 in the state of Veracruz alone, according to the Congressional Research Center report.

Contributing: The Associated Press.