To add to the mystery, scholars have been unable to uncover any trace of the "custom" Pilate refers to - the release of a prisoner at Passover. There is no record of it in Jewish or Roman sources.
Some have theorized that what actually happened was this: a day or two before or after the death of Jesus, a notorious prisoner had been found not guilty and released. The followers of Jesus remembered this and when they told the story of the Passion, they drew a sharp contrast between the release of this hoodlum, and the condemnation of the innocent Jesus.
As time went on, the story was told in a way that brought Barabbas and Jesus together in the same time and place. There they are, side by side. Barabbas, the guilty one, is set free. Jesus, who is innocent, is condemned to crucifixion.
- Little Black Book, Diocese of Saginaw