Nuevo Leon state security spokesman Jorge Domene said the survivor, a member of the Kombo Kolombia band, told police the 18 were blindfolded and driven on dirt roads until they stopped. He then heard the assailants ask fellow band members if they belonged to a drug cartel, shots were fired and the bodies were dumped into a well.
Domene said the survivor, who is being protected by soldiers, was able to reach a nearby ranch and get help. He wouldn't give details on how the man was able to escape.
The man later led authorities to the well where searchers found several bodies, Domene said.
Domene said four bodies first pulled from the well on Sunday have been identified by their relatives, including a Colombian citizen who played the keyboard. Three of them were wearing matching T-shirt with the name of the band.
"The search will continue ... to see how many more bodies may be hidden there," he said.
By Monday afternoon, searchers had pulled 12 bodies from the well along a dirt road in the town of Mina, about 140 miles (225 kilometers) from Laredo, Texas, Domene said.
The bodies recovered showed signs of torture, said a forensic official, who spoke on condition of anonymity because he was not authorized to comment publicly on the case.
It was hard to determine how many more bodies were submersed in the water, he said.
Authorities initially said 16 members of the band Kombo Kolombia and four crew members were reported missing early Friday after playing at a private party attended by about 50 people and held at a ranch called La Carreta, or The Wagon, in the town of Hidalgo north of Monterrey.
But Domene said Monday 18 band members had gone missing. He didn't say how many were crew members and how many were musicians.
The party guests are being questioned and police have yet to determine a motive in the killings, Domene said.
Nuevo Leon state, on the border with Texas, has been the scene of a turf battle between members of the Gulf drug cartel and the Zetas drug gang. The Zetas were hit men for the Gulf cartel until they split in 2010, unleashing their bloody war.
People living near the ranch in Hidalgo reported hearing gunshots at about 4 a.m. Friday, followed by the sound of vehicles speeding away, said a separate source with the Nuevo Leon State Investigative Agency. He also spoke on condition of anonymity because he was not authorized to be quoted by the news media.
The officials added that gunfire is common in the area and said investigators found spent bullets nearby.
Relatives filed a missing persons report on Friday after losing cellular phone contact with the musicians. When they went to the ranch to investigate, they found the band members' vehicles still parked outside.
Kombo Kolombia has played a Colombian style of music known as vallenato, which is popular in working class neighborhood in the city of Monterrey and other parts of Nuevo Leon state. Most of the group's musicians were from the area, except for the keyboard player who is Colombian and had Mexican residency, Domene said.
The band regularly played at bars in downtown Monterrey on the weekend. At least two of the bars where they had played had been attacked by gunmen.
It was Mexico's largest single kidnapping since 20 tourists from the western state of Michoacan were abducted in Acapulco in 2010. Most of their bodies were found a month later in a mass grave. Authorities said the tourists were mistaken for cartel members.
Members of other musical groups have been murdered in Mexico in recent years, usually groups that perform "narcocorridos" that celebrate the exploits of drug traffickers. But Kombo Kolombia did not play that type of music, and its lyrics were about love and heartbreak and did not deal with violence or drug trafficking.
But singers of drug exploits are not the only musicians targeted, said Elijah Wald, author of the book, "Narcocorrido: A Journey into the Music of Drugs, Guns and Guerrillas."
"There is really not correlation. Drug guys hire people to play for their parties and they hire whatever is happening," he said. "Sergio Gomez, the single-most famous singer killed from K-Paz de la Sierra, his big hit was a version of `Jambalaya."'
Gomez was kidnapped and found strangled and tortured in 2007 in the western state of Michoacan, a day after Zayda Pena of the group Zayda and the Guilty Ones was shot in a hospital while recovering from a separate bullet wound in the border town of Matamoros, across from Brownsville, Texas.
Valentin Elizalde, "El Gallo de Oro," was shot to death along with his manager and driver in 2006 following a performance in Reynosa, across the border from McAllen, Texas. Norteno singer Sergio Vega was shot dead in a northern state of Sinaloa in 2010.
"A lot of people are being killed because they're in the wrong place at the wrong time and musicians are some of the people on that list," Wald said.
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