In the summer of 1969, three San Francisco area newspapers received a letter. “Dear Editor,” it began, “I am the killer of the 2 teenagers last Christmass [sic] at Lake Herman and the Girl last 4th of July.” The writer offered corroborative evidence that left no doubt he was indeed present at the two killings mentioned. In subsequent correspondences, the writer identified himself as “The Zodiac” and laid claim to more lives. His were the first such taunting confessions sent to newspapers by a serial killer since Jack the Ripper terrorized London in 1888.

It’s been nearly 40 years since the last confirmed killing, and nearly 30 since a newspaper received any correspondence from anyone claiming to be the Zodiac Killer. Despite exhaustive police work and the aide of two eyewitnesses who survived his attacks, “Z,” as investigators called him, was never caught. Doubts about the authenticity of some letters and the likelihood that the Zodiac took credit for several murders he didn’t commit (a 1974 letter boasted “Me = 37, SFPD = 0,” though most investigators believe it was closer to 7–0) have cooled the trail to the Zodiac as much as time has. The San Francisco Police Department currently has the case filed as “inactive.”

“Zodiac,” the new film from the director of “Fight Club” and “Se7en,” David Fincher, dramatically reactivates — or at least re-enacts — the Zodiac case. Based on a pair of books by San Francisco Chronicle political cartoonist-turned-amateur sleuth Robert Graysmith, the film painstakingly portrays the murders, the fruitless police investigation, and the accompanying press frenzy. Mr. Fincher creates a gorgeously grimy and lived-in 1970s look, with livery skin tones and green-brown shadows that are a far cry from the department store window perfection of Oliver Stone-generation period films.

But during the course of its 160-minute running time, “Zodiac” succeeds at doing something much subtler and more difficult than merely breathing new digital life into yesteryear’s atrocities. “Zodiac” is not a film about a murderer; it’s a film about a community — one that briefly awakens in fear and then quietly slips back to sleep, forgetting both the monsters that threatened it and the guardians who sought to protect it.

The film’s broad yet finely detailed canvas depicts four decades of incident and more than a dozen key players. It’s really two movies in one. The first is a breathless cops-and-reporters drama in which cynical yet committed badge or press-card carrying professionals compete with one another for the public trust. In one corner is Paul Avery, the San Francisco Chronicle’s star crime reporter, played with typically mischievous élan by Robert Downey Jr. In the other is Mark Ruffalo as San Francisco police detective David Toschi. In between is Jake Gyllenhaal as Graysmith, the future Zodiac expert.

In our post-Thomas Harris world, the term “serial killer” gets tossed around like it’s an occupation or religious calling. John Doe, the villain of Mr. Fincher’s “Se7en,” considered himself a force of biblical justice and, thanks to the elaborate sight gags of his grotesque crimes, the audience could see his point. Tyler Durden, the hero of “Fight Club,” was a kind of postmodern nature boy freed from society’s shackles. Who couldn’t, even if just for a fleeting moment, imagine detonating every American debtor’s credit history while listening to the Pixies at top volume as Durden does in the climax of “Fight Club”?

But the multiple murderer at the center of “Zodiac” isn’t an avenging angel, a superman, or a darkly charming aesthete like Hannibal Lecter. Whoever he is, or was, the film says, the Zodiac Killer was a sick, selfishly destructive jerk — an emotional myopic who didn’t care if he was pulling the wings off flies or squashing them, as long as he got some attention. The film’s murder scenes are notable not just for their grindhouse viciousness but for their absolute refusal to grant the fictional Zodiac any of the glory that the real one sought in the press.

With the Zodiac’s actual year-long murder spree concluded and the likelihood of bringing the killer to justice becoming increasingly remote, the charismatically self-destructive Avery and the paradigmatic cool cop Toschi move into the film’s background. Meanwhile, the circumspect Graysmith assumes the rest of the film’s narrative weight with his own open-ended investigation, and “Zodiac” transforms from a taught, unified crime drama into a rather impressionistic and episodic multiyear crusade.

It’s a strange pairing of styles — a tense, expertly crafted re-creation of the recent past married to a brooding drift forward into the present that’s chiefly defined by an elusive and shifting punch list of suspects, witnesses, and cops. Eventually, like David Cronenberg’s 1991 film version of “Naked Lunch” and Gillian Armstrong’s “Little Women” of 1994, “Zodiac” ends up being as much about writing the film’s source book as about the story and events themselves.

Making an active protagonist out of a guy who is essentially a reactive observer is a tough job for any actor, and Mr. Gyllenhaal, who keeps Graysmith uniformly wide-eyed and resolute no matter the year, isn’t quite up to the task. Mr. Fincher, however, attacks the material with relentless visual intelligence and passion. Unlike his previous film, 2002’s “Panic Room,” “Zodiac” contains almost no audacity for audacity’s sake or camera gimcrackery indicating a high-functioning directorial imagination slumming in a dubious script. In “Zodiac” the audience isn’t forced to travel through coffee pot handles. Instead we are directed inside the characters. Every puzzle-piece scene, no matter how incomprehensible its ultimate value will be in unmasking the Zodiac, is at the very least emotionally lucid.

“Zodiac” is also blessed with a deep bench of reliably outstanding character actors filling out the many speaking roles. As police chiefs in competing jurisdictions, Elias Koteas and Donal Logue (almost unrecognizable in a non-party guy role) fearlessly expose how uneasy lies the heart that wears the badge. Clea Duvall’s late-film interrogation takes three script pages to yield a one-word clue and seems like a film unto itself.

This is easily the finest big-studio crime film offering since Curtis Hansen’s “L.A. Confidential.” It’s rare to see an American movie of any kind in which nearly everyone involved is working at the level of excellence that “Zodiac” offers for the majority of its nearly three-hour running time. And it’s even rarer to see a movie with so many sensationalist and prurient avenues of expression open to it take as sober a view of thrill-killing and as compassionate a view of its toll as “Zodiac” does.