Last year, in the Mexican state of Veracruz, authorities announced that they had found a mass grave filled with more than 250 human skulls.

At the time, state prosecutor Jorge Winckler told news station Televisa that “once we have opened all the mass graves in the state, [Veracruz] could become the biggest mass grave in Mexico, or even the world."

On Thursday, Winckler announced that another mass grave had been discovered, again in Veracruz. This time, more than 160 human skulls were found inside.

As many as 40,000 people are missing across Mexico, and about 30,000 are known to have been killed last year. Violence in Mexico is widespread, and Veracruz in particular has suffered bloodshed in recent years. Officials have not revealed the exact location of the gravesite, but speaking to reporters, Winckler said that in addition to the human remains, investigators also found more than 100 ID cards at the site, as well as about 200 items of clothing — raising hopes that some of the victims can be properly identified.

He also said that they are using drones to search for more bodies and will try to match DNA samples from the site with those in a national database of people who have gone missing in recent years.

"The state has become a cemetery, that's the terrible and sad reality,” Winckler told the Wall Street Journal, adding that of 212 municipalities in Veracruz, mass graves had been found in more than half of them. Hundreds of people are missing in the state.

Authorities found this grave in August after receiving a tip, but they did not immediately inform the public because of security concerns. The last time such a grave was found, a group of mothers campaigning for authorities to search for their missing family members were also helped by anonymous tipsters: Two men handed them a hand-drawn map indicating where bodies were buried. The Wall Street Journal reported at the time that the mothers themselves hired diggers to find the grave, and then persuaded authorities to help them.

Drug cartels are believed to be behind the slayings and burials. For years, cartels have operated in Veracruz and elsewhere in Mexico, targeting civilians and police, who are at times complicit with various criminal organizations. The victims may have been in rival gangs, been unlucky witnesses or have refused to collaborate with criminal groups that then tracked them down and killed them in retaliation.

Winckler told the Journal that his office now knows “that government officials collaborated with drug gangs to illegally arrest people of a rival group, torture them and then killed them and disappeared the bodies.” He did not elaborate, but until late 2016, Veracruz's governor was Javier Duarte, who later went on the run after he was accused of embezzling millions of dollars from the state. He is now in jail awaiting trial, as is his former police chief, Arturo Bermudez. Bermudez has been accused of enforced disappearance. The remains from this grave are thought to be about two years old.

Roberto Campa, who was at the time Mexico's deputy interior minister for human rights, told the Journal after the earlier mass grave discovery was announced last year that “something like this can only happen with the complicity of the police."

"There is no way they could have done this for so long without the police knowing,” he said.

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