“Detroit”

**1/2

(R, 2:23)

Director Kathryn Bigelow’s “Detroit” is an artfully frazzled mosaic of suffering, putting the audience through the wringer in the name of truth. At its best the movie evokes a spirit of mournful provocation, and in various, depressing ways, the film speaks to our present-day, rage-fueled American divisions, but “Detroit” is a tough sell for a night at the movies, because it’s about people who never had a chance.

It’s written by Mark Boal, Bigelow’s collaborator on “The Hurt Locker” and “Zero Dark Thirty,” but significant limitations prevent “Detroit” from measuring up to those exceptional standards.

On July 23, 1967, police raided an illegal after-hours bar in Detroit, sparking riots. In the movie Anthony Mackie plays a key supporting character, real-life Vietnam War veteran Robert Greene, caught up in the blind pig raid and the ensuing nightmare. He later described it as worse than anything he endured in Vietnam.

The excruciating centerpiece of “Detroit” concerns what happened at the Algiers Motel, an $11-a-night dive, home to hookers and johns and all kinds of transients. A group of officers, mistaking a starter’s pistol for a sniper’s, turned against innocent suspects in a show of outlandish, illegal force. With the tacit cooperation of state police and the National Guard, the interrogation became a sick game of pretend-killings (for a while) in another room where various battered suspects were being held.

It’s one hell of a difficult sequence to endure. Bigelow does not elide or cut out anything for the sake of going easy on the audience. The film’s audience surrogate, who remains watchful on the margins of this long scene, is real-­life security guard Melvin Dismukes, played by John Boyega of “Star Wars: The Force Awakens.” He’s a singular note of decency amid an indecently cruel scenario.

The key fictionalized character is a venal, sociopathic police officer played by Will Poulter, based on an officer later found not guilty in court by an all-white jury.

This protracted nightmare raises a question: Is there a way to dramatize a gut-grinding scene so that the audience stays rapt instead of checking out? Bigelow knows the answer is yes, but it’s tricky. Here she cuts in and out of the scene for rhythmic variety, picking up other plot strands for a while and returning.

But Poulter is miscast and one-note, and it seriously hurts “Detroit.” As written and as acted, the vicious cop here called Krauss (whom we first see shooting a looter in the back) serves as the embodiment of all law enforcement evil, a paranoid bully, pulling his partners (one willing, the other reluctant) into escalating carnage. Boal has done copious research and read through various accounts of the evening, as recalled by various parties, but the scene is missing something.

Bigelow and cinematographer Barry Ackroyd plow through “Detroit” with an aggressively destabilized camera, familiar if you’ve seen “The Hurt Locker” or “Captain Phillips,” both shot by Ackroyd. It’s a logical choice but a conventional one by now.

A handful of films, from “The Battle of Algiers” to Paul Greengrass’ splendid “Bloody Sunday,” have met the challenge of dramatizing civil unrest and law enforcement outrages memorably. “Detroit” comes close.

The movie includes archival footage, such as Michigan Gov. George Romney decrying Detroit’s “hoodlumism.” A prologue written by Henry Louis Gates Jr. charts the Great Migration and subsequent economic disenfranchisement of countless African-Americans.

“Change was inevitable,” one title card reads in the animated “Detroit” opener, using illustrator Jacob Lawrence’s paintings. It’s a disarmingly folkloric way to introduce the bloodstained mosaic to come.