Mark Ruffalo is in negotiations and Jake Gyllenhaal and Robert Downey have signed onto David Fincher's long-gestating thriller "Zodiac." Phoenix Pictures is producing.

Gyllenhaal will play Robert Graysmith, the journalist and author of the two books, "Zodiac" and "Zodiac Unmasked," upon which the film is based. Downey will play fellow reporter Paul Avery, and Ruffalo would play the San Francisco homicide inspector in charge of the case.

The books revolve around the real-life tale of a serial killer known as the Zodiac Killer, who terrorized San Francisco for 25 years. Graysmith and Avery worked at the San Francisco Chronicle, which the killer used as a conduit to communicate with authorities.

James Vanderbilt is adapting.

The two studios are co-financing the picture. Alli Shearmur and Marc Evans are overseeing for Paramount, which is handling domestic distribution. Lynn Harris, Greg Silverman and Geoff Shaevitz are overseeing for Warners, which is taking international output. Warners is the lead studio on the project.

Phoenix chairman Mike Medavoy, president Arnie Messer and vp production Brad Fischer are producing with Vanderbilt. Fincher's production partner, Cean Chaffin, also is producing.

Ruffalo just wrapped "All the King's Men" for Columbia Pictures. His recent credits include "Collateral" and "Eternal Sunshine of the Spotless Mind." His upcoming releases include "Rumor Has It" and "Just Like Heaven."

Ruffalo is represented by WMA and manager Robert Stein.

Gyllenhaal's credits include "The Day After Tomorrow" and "The Good Girl." His upcoming releases include "Brokeback Mountain" and "Jarhead."

Downey's credits include "Gothika" and "The Singing Detective." His upcoming films include "Kiss, Kiss, Bang, Bang" and the Edward R. Murrow feature, "Good Night, And Good Luck."

Gyllenhaal is repped by CAA and Management 360 and Downey is repped by CAA.