The movie was narrated as a true story by the producer or any who starred in the movie.
The movie is based on a novel written by Stephen King. The novella was titled: Rita Hayworth and the Shawshank Redemption. It wasn’t based on a true story either.
Tommy knew who killed Andy’s wife. He wanted to expose the killer during a court case but he was killed by the Warden before he could do so. The warden did a lot to hinder Andy’s activities and keep him in jail.
Warden needed Andy to keep assisting him if his corruption would continue.