Were plenary indulgences issued during the crusades, as indicated by Dan Brown?

In Dan Brown's novel, *The DaVinci Code*, if my memory serves me correctly, Leigh Teabing asserts that Crusaders were given plenary indulgences to commit unspecified sins with impunity whilst on crusade. I have to admit that I have only seen the film, and not read the novel, but I am assuming that the film follows the novel in this respect. My question is: was this ever official doctrine during the time of the Crusades, if indeed it happened, or was it a perversion of official doctrine, or is it just black propaganda? If the latter, who started it, and when? I understand that indulgences can only be issued for sins already committed.