They didn't find the bodies until 2010, 57 years after they were hastily buried in a disused mineshaft.
Fourteen had died from a single bullet to the back of the skull.
The fifteenth was so viciously garroted that its head was almost severed from its body.
For Ana Maria Galindez, the forensic investigator, it was her first encounter with the work of Comandante Guzman.
But not her last. The Sentinel is the first in an ambitious and enthralling trilogy of novels that explores the Spanish Civil War's savage legacy.
Centre stage is the figure of Leopoldo Guzman, head of the Brigada Especial - Franco's secret police.
Guzman is one of the more remarkable creations of modern fiction.
A monster and an enigma who nevertheless engages the sympathy of the reader.
He is also a man who vanishes from history in the winter of 1953...
Until Galindez unearths his tracks.