The unsolved mystery of “Zodiac,” the serial killer who terrorized California in the 1970s, necessarily remains so in director David Fincher’s film, which chiefly concentrates on the struggles of the investigators most involved in the case. Foremost among these are police detective David Toschi (Mark Ruffalo), San Francisco Chronicle crime reporter Paul Avery (Robert Downey Jr.) and Robert Graysmith (Jake Gyllenhaal), the Chronicle cartoonist who became obsessed with the case and ended up writing two books about it. Fincher and screenwriter James Vanderbilt begin with the initial Zodiac killings in 1969, then settle in with an elaborately detailed account of the investigations that unfolded in various Bay Area police departments and in the offices of the Chronicle. The grisly killings at the outset suggest a horror film/mystery thriller, but it’s soon evident that much of the central action will be in the methodical police procedural mode, augmented with streaks of docudrama and glancing character study. In its earnest (and faintly morbid) realism, Zodiac is a sharp-eyed evocation of California in the ’70s and a haunting portrayal of the heavy toll taken on the most persistent investigators of a sensational and increasingly complex murder case that goes unsolved.