Veteran director Brian De Palma loves reminding American moviegoers just how voyeuristic we are. His sharpest films, like “Dressed to Kill,” “Scarface,” and “Femme Fatale,” are feature-length seductions, orgies of masterful stylistic excess and sinuous suspense. Fittingly, the true-crime story buried within “The Black Dahlia” involves someone who simply longed to be watched: aspiring actress Elizabeth Short, who in 1947 became Hollywood’s most notorious murdered dreamer.

It was a murder tailor made for a De Palma film, a gruesome act of disembowelment and disfiguration, but the action of the “The Black Dahlia” occurs afterward. Reducing the density but not the sprawling feel of James Ellroy’s chunky 1987 novel, the film focuses on the subsequent investigation by a young-buck police detective (Josh Hartnett). Mr. De Palma still manages his own murder set pieces (a surprise shootout, a shadow-play strangling), but his powers of orchestration are subdued; his fascination with artifice is used for dream-factory mood and mythology rather than nail-biting plotting and jaw-dropping camerawork.

The case, named for the victim’s dark hair and monochromatic fashion sense, draws Bucky Bleichert (Mr. Hartnett) and brash fellow cop Lee Blanchard (Aaron Eckhart) into a sultry tangle of mystery. It’s the kind of Big Sleepy whodunit that still feels unresolved by the credits, and Mr. De Palma, who thrives in self-contained cinematic worlds, piles on film noir tropes and lavishes the period detail, like the zoot-suit riot that opens Mr. Ellroy’s work. Mr. Hartnett’s lug voiceovers lead his descent into the amoral underworld; a first-person point-of-view shot in this context evokes the Bogart-Bacall curiosity of “Dark Passage.”

Bucky watches Lee’s efforts on the case warily but with morbid fascination, projecting a sense of entwined fate. The two go toe-to-toe in a charity boxing match for the police association, and the brief prep-room opening dabs another touch of B-noir. But their desires mainly converge on Lee’s girlfriend, Kay (Scarlett Johansson). Kay is less a vixen than another nexus in the film’s shadowy history, a woman with a past involving a mysterious tormentor. Their triangle, more mysterious than torrid, is indelibly defined in one of the movie’s best scenes: seated between the two at the movies, Kay grabs each man’s hand in the dark.

It matters little that Ms. Johansson’s young face atop decked-out styles lends an air of dress-up that dampens the intrigue, because Mr. De Palma’s film only really opens up with the grotesquerie of its second half. Once Lee’s interest in the crime transforms into obsession (somewhat sloppily, as Mr. Eckhart is soon dashing about in puzzling huffs), Bucky finds a new criminal and romantic lead: the eldest daughter (Hillary Swank) of the rich and decadent Linscotts, and a double for the dead girl.

Bucky’s dinner at their house, where Mom (Fiona Shaw) is a riveting, loony drunk and a younger sister draws an obscene cartoon, channels Mr. De Palma’s invaluable sense of camp and parody. The sequence instantly wakes the movie up from its foggy exposition, but it also underlines how the director has been playing more backing harmony to Mr. Ellroy’s obsessive story (the author was himself the son of a murder victim) without really satisfying either man’s vision.

But this funhouse aspect is key because it leads to another vital feature of the film: Bucky’s emerging interest in screen tests and stag films featuring the murdered Short (played by Mia Kirshner, directed in them by an offscreen voice — Mr. De Palma’s).There’s a delicious sense of how the heightened reality of Hollywood movies seems to illuminate decay in the world and in the lives of its practitioners. Elsewhere there’s the perverse spectacle of Mrs. Linscott teetering on a balcony and resembling Bette Davis circa “What Ever Happened to Baby Jane?,” or the priceless detail that her husband jerrybuilt a housing development with discarded movie sets. At the feet of the Hollywood beast is a figure like Short, whose case is the movie’s foundational sin.

What was for Mr. De Palma partly work-for-hire (taken over from David Fincher) becomes a measure of redemption, with the director’s cynically scrawled notes on artistic creation and the mutilated dreamer shining through. “The Black Dahlia” provocatively makes a one-liner out of a high-art clown painting in one scene that evokes the elongated grin carved onto Short’s face. (“I don’t get modern art,” Bucky says.) It’s a signature acknowledgment by a filmmaker who makes such reflexive play and macabre beauty out of murder and ruthless subterfuge.

One almost has to worry for filmgoers lured by Ms. Johansson and Mr. Hartnett, the latest examples of Mr. De Palma’s penchant for box-office cheesecake. That habit has always been another tool of artifice for him — a gamble that sometimes pays handsomely (Rebecca Romijn in “Femme Fatale”), or doesn’t (boy wonder Michael J. Fox in “Casualties of War”). Except for Mr. Eckhart, who disappears too soon, this cast stays lightweight despite being slathered with glamour.

“The Black Dahlia” may be a lesser work for Mr. De Palma, not without ideas but lacking the swing of his thrillers and the confident, shifting storytelling of his best films. But the director does pull out an admirably uncompromising ending that offers little moral solace to any character. At once the most and least satisfying thing about the movie, that at least should leave a lasting cigarette burn on the mind.