Prosecutors in Mexico have arrested the mayor of Teuchitlán in western Jalisco state as part of their investigation into a nearby cartel training site.

The mayor, José Murguía Santiago, is suspected of colluding with the New Generation Jalisco Cartel (CJNG), which he has denied.

The investigation was launched after activists discovered bone fragments and hundreds of discarded shoes, backpacks and items of clothing at the Izaguirre ranch outside the town in March.

Rights groups said that they feared the ranch had been used as an “extermination camp”, where people were forcedly recruited and trained, and those who refused were tortured and killed.

The discovery by people searching for their missing relatives of what appeared to be evidence of mass killings at the site shocked the country, where cartel violence is rife.

Mexico’s Attorney-General Alejandro Gertz gave a news conference last week updating journalists on the federal investigation into the ranch.

He confirmed that the site had been used as a training centre for recruits of the Jalisco New Generation Cartel, one of the most feared and powerful transnational drug trafficking gangs, which has its power base in Jalisco.

However, he said that there was no evidence that it had been used as an extermination and cremation site.

According to the attorney-general, bone fragments discovered there were not recent and forensic tests suggested that the fires lit at the ranch would not have been hot enough to dispose of human remains.

Gertz’s statements caused anger among “searchers”, the name given to relatives looking for the more than 120,000 people who have been reported missing in Mexico over the past two decades.

They said that his news conference raised more questions than it answered and failed to address to whom the many abandoned shoes found at the ranch belonged and what had become of those people.

Gertz insisted that the authorities would continue looking into whether there had been any collusion between the CJNG and local officials.

The arrest of Mayor Murguía Santiago is part of that ongoing investigation.

Prior to his arrest, the mayor had said that he had nothing to hide. “If they want to investigate me, let them, I’m clean and willing to say what I know,” he told local media.

But prosecutors allege that he knew of the existence of the training centre and did not act on that knowledge.