MEXICO CITY (AP) — A horrifying execution video circulated Wednesday on social media may have recorded the last moments of five kidnapped young men, and has transported Mexico back to the darkest days of drug cartel brutality in the 2000s.

Prosecutors in the western state of Jalisco say they are investigating the video, and relatives of the missing group of young friends told local media that their clothing resembled that worn by the men in the video.

The most horrifying thing is not just the pair of bound, inert bodies seen lying in the foreground. It is the fact that the youth seen bludgeoning and apparently decapitating another victim appears to be himself the fourth member of the kidnapped group of friends.

The fifth member of the kidnapped group — young friends who had traveled to attend a festival in the city of Lagos de Moreno in Jalisco state — may be the body police found inside a burned-out car in the area. The young men went missing Friday in an area known for cartel violence, and authorities have mounted a massive search for them.

Luis Méndez Ruiz, the Jalisco state attorney general, said Tuesday that the men seen in the video “could be the five men who are being searched for.”

“This video and the information that was made public on a social media platform is now part of the investigation,” Méndez said. The clothing worn by the men in the video also resembles a photo of them alive, but bound, that was released earlier.

The video features a text written over the image that says “Puro MZ,” an apparent reference to El Mayo Zambada, the leader of a faction of the Sinaloa drug cartel. But it was unclear who was responsible for the video.

If confirmed, the video — which shows someone off-screen tossing the youth a brick, so he can bludgeon the victim with it — would revive memories of the most horrifying instances of drug cartel brutality, in which kidnap victims were forced to kill each other.

In 2010, one Mexican cartel abducted men from passenger buses and forced them to fight each other to death with sledgehammers.

That tragedy came to light in 2011, when authorities found 48 clandestine graves containing the bodies of 193 people in the northern border state of Tamaulipas. Most had their skulls crushed with sledgehammers, and many were Central American migrants.

It was later revealed the victims had been pulled off passing buses by the old Zetas drug cartel, and forced to fight each other with hammers or be killed, if they refused to work for the cartel.

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