As MacDonald points out, Mark provided an important clue to his readers that his story was fictional which no one could miss: he called the prisoner "Barabbas." Barabbas means "the son of the father" 1. Thus, Mark has Pilate set up a contest between two "sons of the father", one real, the other not. Of course, this unlikely contest probably only took place in the creative imagination of Mark, who wanted to create for his readers a parable teaching the goodness of Jesus and the evilness of his persecutors.
An Unlikely Practice
MacDonald notes that there is no evidence independent of Mark that it was ever the custom at feasts for the Romans to release a prisoner requested by the Jews. Furthermore, such a practice would make no sense, and would be quite foolish: it's not likely, for example, that the Romans would release a prisoner accused of murdering soldiers. Thus, this "practice" of releasing a prisoner probably existed only in the imagination of Mark, who just needed to have two men to have a contest between one good man and a bad one, similar to the one which occurred in Homer; Mark already had the first man on the stage--Jesus, and through an artless contrivance he brought on the second one
I always suspected that this story did not take place and is meant as a parable. However, it has been used through the ages to cast aspersions upon Jews. I don't believe that any historian mentions a Bar-Abbas.