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FBI agent Kate Macer (Emily Blunt) thought she was doing the right thing when she accepted an assignment to battle drug cartels at the U.S.-Mexican border. But it’s not long before she’s as nervous about her colleagues in the field as she is about the criminals they’re presumably targeting.

Matt Graver (Josh Brolin), leader of the inter-agency operation, seems reluctant to share any information with her that can’t be categorized as need-to-know. Even shadier is Alejandro (Benicio Del Toro), a “sicario” (hitman) who is said to have once been a prosecutor in Mexico but doesn’t seem the least bit concerned with legal niceties.

With no one to trust, Macer looks on in astonishment as bullets fly, blood spurts and bodies fall. If this is the war on drugs, she’s a serious candidate for conscientious objector status.

“Sicario,” in Hollywood parlance, is “Traffic” (2000) meets “Zero Dark Thirty” (2012). Like the former, it portrays the drug trade as a hellish maze. Like the latter, it questions the morality of lawlessness in the name of justice.

Working from a screenplay by Taylor Sheridan, director Denis Villeneuve doesn’t flinch from exposing the dark side of law enforcement. In spirit, “Sicario” is a sequel to his 2013 film “Prisoners,” which also explored the tragic repercussions of violence. The new film reaches its apotheosis in a scene of retribution that casts Alejandro in an unsparing but revelatory light — and dares the audience to identify with him.

The performances are outstanding. Blunt, who almost upstaged Tom Cruise in last year’s “Edge of Tomorrow,” is the still center around which the narrative revolves. In his best role since “No Country for Old Men,” Brolin delivers a portrait of cool, smiling menace. And Del Toro, who took home an Oscar for his turn as a soulful cop in “Traffic,” is unsettlingly enigmatic.

Far from being just another crime story, “Sicario” is cinema at its most ambitious.

What “Sicario” • 3½ stars out of four Run time 2:01 • Rating R • Content Violence, grisly images, language