Starring Jake Gyllenhaal, Mark Ruffalo, and Robert Downey Jr. Rated 14A.

Zodiac, a serial-killer movie about movies more than murders, tackles even scarier issues-mainly in its view of media as the poisoned water in which modern sociopaths swim.

Zodiac is front-loaded with two brutal incidents in 1969 but then pretty much settles into a police procedural that would be reassuring-as calm as a 160-minute episode of Law & Order, let's say-except that the order keeps unravelling and law has hardly anything to do with it.

The tale is closely based on events that dominated headlines in the San Francisco Bay area in the late 1960s and on two books written about them by Robert Graysmith. Here, portrayed by Jake Gyllenhaal, the author inserts himself even more vehemently than is usually the case in real-life stories. Sure, Woodward and Bernstein may figure prominently in All the President's Men, but Graysmith was a political cartoonist, for cryin' out loud, and the audience, like his coworkers, is initially befuddled by his growing obsession with the killer who calls himself Zodiac.

The Chronicle writer most involved with the case is reporter Paul Avery, played with the kind of sly, slightly wounded confidence that has turned Robert Downey Jr. into one of the most consistently entertaining actors working today.

This fearlessly self-destructive fellow is slow to warm to Graysmith's interest and to his squeaky-clean way of living. "We weren't all Boy Scouts," Avery says, underlining his statement by snorting from a little vial of coke he carries everywhere.

Deftly directed by Bay Area native David Fincher, who eschews the baroque stylishness of Seven and Fight Club for '70s grit in a yellow-gray palette, the movie takes us swiftly through the Watergate era, with "Helter Skelter" paranoia increasing as the decade drags on and the Zodiac investigation becomes more tangled and confusing. Even when a prime suspect emerges (played by bulky John Carroll Lynch), the case is full of contradictions, especially because Zodiac messes up, breaks his patterns, and keeps claiming credit for other people's crimes. The only sure thing is that he likes to read about himself in the paper. Zodiac is more into self-marketing than killing; he even has his own logo.

Another major player is homicide detective Dave Toschi (Mark Ruffalo). "So much for due process," Toschi says, taking a smoke break from the vigilante antics of Dirty Harry, screened for the SFPD.

As the movie proceeds, there are increasing references to film-noir and true-crime exploitationals, even as new technologies, like "telefaxes", promise to improve communication and, with it, the police work that here is hampered by bureaucratic entanglements and failures of imagination.

Fincher fails to explain why Graysmith, who lost his family over the case, became as unhinged as Richard Dreyfuss making mashed-potato mountains in Close Encounters of the Third Kind. But he hints at our own weaknesses as consumers of infotainment and points to the pathologies that make us vulnerable to tabloid horror, and our simultaneous drive to understand it.