The attackers burst into a house the Spaniards had rented on the outskirts of Acapulco, in a low-key beachside area, and held 12 Spanish men and women and one Mexican woman at gunpoint before dawn on Monday.
They tied up the six men with phone cords and bathing suit straps and then raped the six Spanish women, said Acapulco Mayor Luis Walton at a press conference later Monday.
"This is a regrettable situation, and of course it is going to damage Acapulco," said Walton. The once-glittering resort that attracted movie stars and celebrities in the 1950s and 60s has already been battered by years of drug gang killings and extortions, but except for a very few incidents, the violence largely has not touched tourists.
Walton said he believed, but wasn't sure, that the assailants in Monday's attack didn't belong to a drug gang.
"From what the attorney general has told me, I don't think this was organized crime, but that will have to be investigated, we don't know," Walton said.
Mexico's Foreign Relations Department issued a statement saying it regretted the attack, and suggesting it was not drug-cartel related.
"This is a common crime, and thus up to now, the investigations are being carried out by local authorities and they will be the ones to provide information," the statement said.
In Mexico, federal authorities investigate drug-related crimes.
Security and drug analyst Jorge Chabat said that, after years of drug gang activity in Acapulco, the distinction may be merely semantic.
"At this point, the line between common and organized crime is very tenuous, there are a lot of these gangs that take advantage of the unsafe situation that currently exists, they know the government can't keep up," Chabat said. "Everything points to this being organized crime, because several gangs have operated there for years ... it's probably not the big cartels, but there are smaller groups that carry out crimes on a permanent basis."
The Spanish Embassy in Mexico City said the victims were receiving consular assistance.
The attackers gained access to the house because two of the Spaniards were in the yard and apparently were forced to open the door, Walton said. The house is on a more isolated stretch of beach east of the city.
The victims were "psychologically affected" by the attack and received treatment, the mayor said. The lone Mexican woman in the group was not raped.
Guerrero state Attorney General Martha Garzon Guzman said witness descriptions of the attackers were more difficult to obtain because they wore masks.
Spain's Foreign Ministry had already issued a travelers advisory on its website for Acapulco before the Monday attack, listing the resort as one of Mexico's "risk zone," though not the worst.
"In Acapulco, organized crime gangs have carried out violence, though up to now that has not affected tourists or the areas they visit," the advisory states. "At any rate, heightened caution is advised."
The attack came just three days after a pair of Mexican tourists returning from a beach east of Acapulco were shot at and slightly wounded by members of a masked rural self-defense squad that has set up roadblocks in areas north of Acapulco, to defend their communities against drug gang violence.
The vigilantes say the Mexican tourists failed to stop at their improvised roadblock.
Walton said the city was already contemplating ways to revive the city's image.
"We have to look at an advertising campaign to say that not everything in Acapulco is like that," Walton said. "This happens everywhere in the world, not just in Acapulco or in Mexico."
Associated Press writer Bertha Ramos reported from Acapulco, and Mark Stevenson reported from Mexico City.