After forever altering the look and feel of serial-killer thrillers with 1995’s “Seven,” director David Fincher returns to the genre with a radically different purpose and result in “Zodiac.” Based on the nonfiction book by Robert Graysmith about the infamous San Francisco murderer whose crimes were never officially solved, the movie is a thriller in subject matter only.

Instead, Fincher and screenwriter James Vanderbilt, who spent more than a year researching the script, have made one of the most detailed, factually scrupulous crime dramas to ever come out of a Hollywood studio. Taking more than a minor cue from “All the President’s Men,” “Zodiac” spends practically every minute of its 2 1/2 hours consumed with the particulars of the investigation that began with the shootings of two teenagers on July 4, 1969 – and stretched on for nearly two decades.

The story splits its time between two main arenas: the newsroom of The San Francisco Chronicle, where Graysmith (played by Jake Gyllenhaal) worked as an editorial cartoonist and helped crime reporter Paul Avery (Robert Downey Jr.) decipher the first of the killer’s missives to the paper; and the San Francisco police department where homicide inspector Dave Toschi (Mark Ruffalo) and his partner William Armstrong (Anthony Edwards) raced to catch the murderer as the media frenzy – and public panic – around them snowballed.

Aside from a couple of re-enactments of the lunatic’s murders (including the disturbing daylight assault of a young couple relaxing near a lake), “Zodiac” never leaves the side of the increasingly obsessed men whose desire to nab the killers grows more voracious the longer he runs free.

Shot by cinematographer Harris Savides (“Elephant”) in a flat, even lighting scheme that beautifully evokes the time period without pulling your attention, “Zodiac” has a purposely nondescript style that makes it seem as if Fincher was apologizing for all the show-off camera antics in “Panic Room.”

But the straightforward approach is crucial, because the movie is constantly doling out so much information – so many names and places and theories to keep track of – that it borders on the overwhelming. Occasionally, it’s a little dull, too: Fincher gives every lead and dead end the same emphasis, wanting to make us feel the density and texture of the investigation, but there are times when you wish he’d been a little more judicious and shaved a few minutes.

Then again, “Zodiac” is the kind of movie that, if you like it the first time around, gets even better on second viewing – precisely because of Fincher’s decision to burrow so deeply into procedure. Even more than a recreation of a real-life crime and its cultural aftermath, this is an obsessive movie about obsessed men.