"Zodiac" is a departure for director David Fincher, he of such unremittingly dark and visually eye-popping thrillers as 1995's "Seven," 1999's "Fight Club," and 2002's "Panic Room." Less a sensationalized serial murder picture than a talk-heavy, scrupulously researched investigation procedural, the film delves into the frustrating decades-long hunt for the Zodiac, an enigmatic killer who terrorized the San Francisco area in the late 1960s and early '70s. Clocking in at a whopping, epic-sized 160 minutes, "Zodiac" shows some signs of choppy post-production editing; the first cut was allegedly an hour longer, and it shows, as characters abruptly drop in and out of the story and certain relationships remain undefined. Still, this is an absorbing, hugely ambitious, richly textured entertainment, and one of the better releases of a still-early year.

Based in part on the book by former San Francisco Chronicle cartoonist Robert Graysmith, "Zodiac" primarily follows three real-life people whose lives were progressively haunted by the seemingly unsolvable mystery surrounding the identity of the self-proclaimed Zodiac Killer: hard-living newspaper reporter Paul Avery (Robert Downey Jr.), assigned to the story after the Zodiac began sending cryptic letters, warnings and codes to the San Francisco Chronicle; SFPD Inspector David Toschi (Mark Ruffalo), prime investigator on the case; and Robert Graysmith himself (Jake Gyllenhaal), a lowly cartoonist who becomes interested and later obsessed with finding out the truth long after the murders and letters subside. With only circumstantial evidence to go on, however, these three people's hope for resolution begins to dim as the years pass them by.

In what could almost be a companion piece to 1999's "Summer of Sam," Spike Lee's masterful exploration into the sweltering season of the Son of Sam killings in 1970s New York City, "Zodiac" is likewise a fascinating look at a particular time and place as it relates to the fear and paranoia of a madman on the loose. While the former film centered on fictional characters who just happened to be living in the area during the Son of Sam reign, director David Fincher zeroes in on those most closely related to the investigation into the Zodiac. Everyday citizens of the Bay area, as well as the families of the victims, are either shortchanged in the grand scheme of things or were the victims themselves of heavy cutting, but that's okay. "Zodiac" isn't so much about them, or the murders themselves, as it is about professional men swept up, encumbered and ultimately almost destroyed by their inability to piece the puzzle together in order to figure out whodunit.

Fincher's unusually low-key approach to the material works here, because the story he has set out to tell favors the human condition over slam-bang action sequences. The setpieces of the crimes are detailed as well (they all occur during the movie's first act), and the results are nothing short of chilling in their stark, straightforward brutality. The opening scene, in which a young couple are shot at a lover's lane on the Fourth of July, unforgettably jump-starts the story and is made all the more indelible by the brilliant use of the song "Hurdy Gurdy Man" by Donovan. The depiction of the masked Zodiac's stabbings of another couple picnicking alongside Napa County's Lake Berryessa is equally effective, with the lone sound of a nearby woodpecker minimalistically ratcheting up the tension. Finally, a scene in which a mother (Ione Skye) narrowly escapes with her baby from the clutches of a man who might or might not be the Zodiac is shiver-inducing.

Other than these fleeting steps into horror territory, "Zodiac" is very much an expositional drama that journeys in-depth into an investigation that drags on for over a decade before threatening to go cold for good. First and foremost an actors' film, the ensemble cast is uniformly superlative. Jake Gyllenhaal's (2005's "Brokeback Mountain") rock-solid portrayal of Robert Graysmith is the one the viewer latches onto the most, as his career and home life with wife Melanie (Chloë Sevigny) and three children suffer at the expense of a fixation over the Zodiac case. Graysmith eventually went on to pen two successful books on the subject, but his personal story is a sad one that shows what he had to, at least temporarily, sacrifice.

Mark Ruffalo (2006's "All the King's Men") is quietly compelling in the role of police detective David Toschi, the toll the case takes on him staying a largely interior one. And as self-destructive reporter Paul Avery, Robert Downey Jr. (2006's "A Scanner Darkly") mesmerizingly layers his role with a buoyant personality masking the emotional wounds and feelings of regret underneath. Further mention should go to John Carroll Lynch (2004's "Catch That Kid"), excellent as number-one suspect Arthur Leigh Allen, and Chloë Sevigny (2003's "Shattered Glass"), doing a lot with little as Robert's neglected wife Melanie.

Deliberately paced but tantalizingly immersive for every second, "Zodiac" earns the right to slowly simmer as all avenues of the story and investigation pull the viewer in. Kudos, too, to director David Fincher's and screenwriter James Vanderbilt's (2003's "Basic") devotion to historical accuracy; they don't simply throw in exploitation or excess to pep things up, trusting that the facts can often be more intriguing than fiction. In turn, suspense arrives not from violence or cheap jump-scares, but from the nagging notion that these horrible crimes may never be solved, leaving the perpetrator a free man who has managed to elude authorities with the same ease as getting out of bed in the morning. Truth be told, "Zodiac" is so riveting and rife with further material left untapped that keeping its initial three-hour-plus length probably wouldn't have been such a horrible idea at all.