— The headless torsos of 43 men and six women were found early Sunday along a highway between the U.S. border and the northern Mexican city of Monterrey, the latest in an escalating series of horrific mass killings among warring drug gangs here.

The dead were discovered just after midnight by a military patrol, according to officials in Mexico’s Nuevo Leon state. Some were in plastic garbage bags, others were scattered about in the dust, as if dropped from the back of a dump truck. All were without heads and extremities, officials said.

Scrawled on a banner left with the 49 victims was a message from the Zetas drug gang asserting responsibility for the killings, said Jorge Domene, a spokesman for the state government. The message included threats to Mexican authorities and the Zetas’ main criminal rivals, the Gulf cartel and Sinaloa cartel kingpin Joaquin “El Chapo” Guzman.

It was signed by the Zetas’ alleged top leaders, Miguel Ángel Treviño Morales, known as “Z-40,” and Heriberto “El Lazca” Lazcano Lazcano, Domene said.

“This was an act of total barbarity,” Domene said by phone. “It’s part of the same thing we’ve been seeing all over the country.”

With Mexico’s presidential election just seven weeks away, the country’s hyper-violent mafias have been seemingly engaged in a gruesome game of one-upmanship in recent weeks.

On Wednesday, 18 mutilated bodies turned up in abandoned vehicles along a highway near Guadalajara, Mexico’s second-largest city. Local news reporters said many of the victims had nothing to do with the drug trade.

Zetas gangsters also took responsibility for those killings, which were apparently in retaliation for the murder of 23 of the cartel’s members in the border city of Nuevo Laredo this month.

Those victims were found ­May 4, with nine hung from a bridge in the center of town and the 14 severed heads of the others left in coolers outside city hall.

“All of these acts are part of a media strategy to get attention,” said Javier Treviño, the former lieutenant governor of Nuevo Leon.

Identifying the dead found Sunday will be difficult given the absence of identifying features, Mexican officials said, but they seemed eager to suggest that the 49 were not likely to have been innocent victims. Some of the torsos had tattoos of Santa Muerte, or Saint Death, the iconic grim-reaper figure that is worshiped widely in Mexico’s criminal underworld, they said.

Forensic work will also be difficult because the victims appeared to have been dead at least 48 hours, and many of their bodies were covered in mud and dust. The site where they were dumped is near the tiny town of San Juan, about 75 miles south of the U.S. border town of Roma, Tex.

Authorities searched the area and found no other victims, Domene said, emphasizing the possibility that the dead were killed in another state or location and driven to the site to be dumped.

“They left them there so they would be seen,” he said.

None of the torsos had bullet wounds, and the bodies appeared to have been transported along the unpaved back roads that wind among local farms and ranches, Domene said. A large deployment of Mexican federal police and soldiers was sent to search the area.

Mexico’s presidential candidates have campaigned in the area on pledges that they will halt the violence that has turned Monterrey, the once-safe industrial capital of northern Mexico, into a gangster horror show.

An arson attack carried out by Zetas gunmen on a casino in the city killed 52 in August.

Monterrey’s renowned business community has been besieged by kidnappers and extortionists, and the threat to the city is so great that it risks becoming a drag on the entire Mexican economy, experts warn.