The film will be directed by David Fincher. Filming begins in August.
Jamie Vanderbilt wrote the screenplay, which was inspired by the books by Robert Graysmith, entitled Zodiac, and Zodiac Unmasked: The Identity of America’s Most Elusive Serial Killer Revealed.
Gylllenhaal will play Graysmith, a reporter for the San Francisco Chronicle who is investigating a series of murders. The Zodiac terrorized the San Francisco area from 1966-78, committing at least 37 murders and documenting his exploits in taunting letters sent to the Chronicle.
Graysmith believes he identified the killer, but competing investigations and other circumstances prevented the suspect, who died in 1992, from ever being apprehended.
Ruffalo will portray the lead investigator in the case, Downey a reporter.