In this article I present a paradigmatic case to exemplify how cross-examining sources constitutes one of the core pillars of our research work. I argue that a primary source, which can be easily regarded as a piece of evidence that is beyond accidental or intentional alterations, that can only convey veracity, and is, therefore, broadly speaking, authentic, must also be cross-examined. There is no such thing, or there should be no such thing as a primary source invested with an aura of sanctity, exempt from the need for verification. The critical examination of sources, that process of comparing and contrasting information, is to our profession what the microscope and reagents are to the natural sciences. Cross-examining pieces of evidence constitutes the crux of all research work, as a piece of evidence is, as a rule, an incomplete and imperfect piece in the puzzle. But no matter how incomplete and imperfect they may be, those fragments of evidence are all we have to recreate the past. Tracing the origins of a tiny detail, such as a handwritten note in a mostly typewritten archival document, can be the thread that gives us the answer to a series of questions. From now on, all meticulous research work will consist of pulling this thread so that it may lead us to new and better explanations. That tiny detail is what we have been waiting for in order to finally piece together the pattern. This article is made from the analysis of documents from the Guatemalan Army, documentary collections from the Historical Archive of the National Police, newspapers, declassified documents from the United States Government, and oral sources.