Enric Marco was exposed shortly before he was due to share a platform at the camp with then Prime Minister Jose Luis Rodriguez Zapatero.
Madrid-based historian Benito Bermejo, an expert on Spain's deportees, became interested in Marco after meeting him at a conference in 2002. Bermejo found his story especially intriguing as Enric Marco claimed to have been imprisoned in Flossenbuerg, a camp in Bavaria and an unusual destination for a Spanish deportee.
Bermejo read all the versions he could find of Marco's past, starting with his claim that he had been an anarchist forced to flee to France from his home city of Barcelona after the Spanish Civil War had been lost.
"I was curious, interested, but then I became very perplexed."
"[Marco's] version of events changed each time he told it, both about the camp and how he had got there," Bermejo told the BBC.
Benito Bermejo also found it mysterious that on the few occasions he tried to talk to him face to face, Marco did not want to discuss his experiences in Germany.
As head of the Amical de Mauthausen, Marco showed a penchant for high-octane speeches packed with horrific details of life in Flossenbuerg.
He moved several MPs to tears when addressing Congress on International Holocaust Remembrance Day in January 2005.
Searching the foreign ministry archive, the historian found an official request from army command in Catalonia for information on Marco's whereabouts, as he had failed to present himself for compulsory military service in 1943.
The foreign ministry replied that Marco was at that time employed by the Deutsche Werke naval shipyard in Kiel, northern Germany.
Far from fighting fascism, he had signed up as one of 20,000 Spaniards who worked for the Third Reich under a 1941 agreement between Franco and Hitler.
"So now I know that Marco was not a deportee, that he went to Germany voluntarily and that there is something strange going on," says Bermejo.
But he still had doubts over the extent of Marco's deception because some volunteer workers who got into trouble did end up in concentration camps.
Marco was briefly imprisoned in Kiel but never convicted, let alone sent to a camp.
For months Bermejo sought an explanation from him. Then, with the 60th anniversary event at Mauthausen days away, he sent a report to the prime minister's office and the Amical association. And he waited.
"What more could I do? I decided that going public with what I knew would be a kind of declaration of war and very controversial at that moment."