The first suspect the police traced was a German man who lived a solitary life, apparently that of a pensioner, in a home in Coin, a picturesque town near Malaga.
Here, he received weapons from eastern Europe and used what police called "a sophisticated range of equipment" in a concealed basement to remove serial numbers and boost the weapons' potency.
The Spanish investigators were tipped off about this man by German police, who also wanted him in connection with an arms cache found in Hannover several years ago.
Next in the chain was found to be another German man, who is accused of having stored the customised weapons, along with the group's collection of Nazi artefacts.
The third leader of the group to be arrested was a British man who is alleged to have been the distributor of the illegal weapons, using high-powered cars with concealed compartments to deliver guns to members of the local underworld. He is also accused of using false passports when making gun runs.
The probe was launched in response to an alarming rise in the number of violent incidences of score-settling or robberies between criminal gangs in the Costa del Sol and the Bay of Gibraltar areas.
The three unnamed men are accused of criminal organisation, the trafficking of weapons, ammunition and drugs, as well as falsifying documents.