The state attorney general has not yet released any information on the suspects or where they're being held, prompting demonstrations by family members concerned about their whereabouts.
Fifty investigators have been dedicated to the case.
The six women were among 14 people victimized by hooded gunmen who burst into a beach bungalow in the resort town before dawn February 4. There are seven suspects between the ages of 20 and 30, lead investigator Marcos Juarez said.
In addition to the rapes, the men stole cell phones, iPads and tennis shoes from the victims, investigators said.
Investigators believe the victims bought drugs from one or more of the suspects a day or two earlier, and that the victims knew the suspects, Juarez said last week.
The Spanish nationals range from ages 20 to 34 and are under the protection of Mexican authorities in Mexico City.
Seven men who were with the group were tied up with cell phone cables and bikini straps while the gunmen assaulted the six women, officials said.
A seventh woman, a Mexican, was spared because of her nationality, Guerrero state Attorney General Martha Garzon said in a radio interview Wednesday.
"She has said that she identified herself to the men and asked them not to rape her," Garzon told Radio Formula. "And they told her that she had 'passed the test' by being Mexican, and from that point they don't touch her."
The gunmen's motive was robbery and "to have some fun," as they saw it, Garzon said. They do not appear to be a part of organized crime, officials said.
Military checkpoints have been set up to apprehend the suspects.
As they sift through evidence, investigators have cordoned off the area around the bungalow, which is in Playa Encantada.
Last year, the city of Acapulco attracted half a million tourists -- most of them Mexicans.
Mexico's Foreign Ministry said in a statement that the Spanish tourists received consular aid after the incident.
The U.S. State Department says "resort areas and tourist destinations in Mexico generally do not see the levels of drug-related violence and crime reported in the border region and in areas along major trafficking routes."
But the agency adds that resort city bars, including those in Acapulco, can be "havens for drug dealers and petty criminals."
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