Rodriguez co-directed the 2005 movie with Miller, who also penned the screenplay, based on his graphic novel. The cast of the original film included Clive Owen, Mickey Rourke, Bruce Willis, Elijah Wood, Jessica Alba, Brittany Murphy, Carla Gugino, Benicio Del Toro and Josh Hartnett.
Monahan, who won an Oscar for writing the script for 2006's "The Departed," is polishing a "Sin City 2" draft recently written by Miller, the entertainment industry trade newspaper reported, cited unnamed sources close to the project.
Rodriguez said last month at Comic-Con in San Diego that financing for the sequel had been secured and shooting would begin as soon as a screenplay was finished.
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