You're referring to the 2007 film "Zodiac" directed by David Fincher!

The film is based on the true story of the hunt for the Zodiac Killer, a serial killer who terrorized the San Francisco Bay Area in the late 1960s and early 1970s. The story follows three men - Robert Graysmith (Jake Gyllenhaal), a cartoonist and amateur detective; Paul Avery (Robert Downey Jr.), a journalist; and Dave Toschi (Mark Ruffalo), a police detective - as they become obsessed with solving the case.

Here's a feature on the film:

Zodiac Release Year: 2007 Director: David Fincher Starring: Jake Gyllenhaal, Robert Downey Jr., Mark Ruffalo, Anthony Edwards, and John Carroll Lynch

Hope you enjoy the film!