At least four inmates died after their throats were cut in a riot at a Mexican prison in the southwestern city of Acapulco, police said, in the latest outbreak of grisly violence to roil the country, Reuters reported.
Acapulco is the biggest city in Guerrero, one of Mexico's most lawless states and a center of opium poppy production that has been a major concern to U.S. officials.
The prison chaos was particularly embarrassing to Mexico as it came the same day U.S. Department of Homeland Security (DHS) Secretary John Kelly was visiting Guerrero.
Guerrero state police said in a statement they heard shots early on Thursday that came from the maximum security wing of the jail. At least four male inmates in the wing died and had their throats slit, police said.
However, a law enforcement official, speaking on condition of anonymity, told Reuters at least six prisoners had died and that four of them had been decapitated.
Kelly, one of the main links between the Mexican government and the Trump administration on migration and security cooperation, arrived in Mexico on Wednesday and held meetings with top officials including President Enrique Pena Nieto.
Two Mexican officials said Kelly would be able to observe Mexico's efforts to eradicate poppies during his visit and two other officials said he had gone to Guerrero on Thursday.
A U.S. embassy official could not provide details about the Homeland Security chief's activities on Thursday.
The killings in Acapulco are the latest example of an upsurge in violent crime that has turned 2017 into one of the bloodiest years in the country's modern history.
Acapulco, one of Mexico's most famous beach resorts, was once a playground for Hollywood stars. However, in recent years it has been roiled by vicious gang warfare, and is now ranked one of the most murderous cities in the world.