Dan Brown studied art history. Within art history is the secret source of Brown’s information to the Holy Grail legends, because art is also an allegorical language, alike to Egyptian hieroglyphs.
Dan Brown didn't plagiarize. The same results and information can be found by anybody researching the art history of the symols of the Holy Grail legends.
There is certain symbology used in the movie Da Vinci Code that is false, such as the Jewish star at Roslyn Chappel. And they used other symbology that is true and tell histories that only grandmasters and ranked royal families know. Stories such as the identity of the real Mary Magdalene in history.
Dan Brown didn't tell us in words or directly, he told us in coded in symbols in the last two scenes of the Da Vinci Code movie. I am an art historian and spent 7 years studying the same scope of symbols and found them all over the beautiful Victorian houses of Haight Ashbury San Francisco, where I live.
Some of my research is in video book form at www.codedhaightashbury.com (New book with more advanced research to be released in a few months. )A picture tells a thousand words, a symbol tells an epic ;-)
Dan Brown did the world a favor by waking people up from a brainwashed stupor from censored media that most of the world still don't know they are deeply in. Holy Blood Holy Grail authers do deserve respect for their efforts, but their line of research is a dead end. Plus they did benefit big time from the notoriety of the court case against Dan Brown. This is what I have found in this scope of research.
Dan Brown did it using popular electronic media, fighting fire with fire. Very hip and cool. Maybe Dan Brown is a grand master undercover.