It has taken more than a year for “Did You Wonder Who Fired the Gun?,” Travis Wilkerson’s great documentary of mourning and reckoning, to reach general audiences—it premièred at Sundance in January, 2017, and opens at Film Forum on Wednesday. (I saw it last March, at the True/False Film Festival, and wrote about it here at the time.) Though it would have been better for audiences to see it sooner, some of its most prescient elements now virtually pierce the screen with real-world affinities. It’s an extraordinary personal documentary, in which the filmmaker’s firsthand experience is shown to reach a vast span of politics and a surprising depth of history. The idea that drives Wilkerson’s film is a powerful one, namely, that everyone’s private life and family traditions are inseparable from politics, from ideology, from history—and that the unwillingness to do the practical, intellectual, and emotional work to find out the specifics of those connections and to face up to them is both a form of privilege and a perpetuation of injustice.

Marching in Los Angeles to protest the acquittal, in 2013, of Trayvon Martin’s killer, George Zimmerman, Wilkerson, who is white, decided to investigate a bit of ugly family lore: the rumor that his great-grandfather Samuel Edwin (S. E.) Branch, a grocer in the small Alabama town of Dothan, killed a black man in the store. Wilkerson’s project is literally documentary: he pursues the documents that can be found, including two newspaper clippings, from October, 1946, that report on the incident—Branch’s shooting of the man, Bill Spann, and Spann’s death, which resulted in Branch being charged with first-degree murder. (Branch nonetheless never stood trial.) Wilkerson gets hold of a copy of Spann’s death certificate, which is specific about the cause of death (two bullet wounds), Spann’s age (forty-six), the name of his widow (Lily), and the hospital in which he died, but offers no information about Spann’s life or his place of burial.

Above all, Wilkerson gathers family photographs of Branch. Especially interesting are the two extant home movies of Branch, taken by a “photo buff” relative: one in black-and-white, also from October, 1946 (whether before or after the killing isn’t clear), and one in color, from 1953. Wilkerson visually analyzes the filmed images of Branch and compares them, in voice-over discussion, with all that he knows about Spann (he is particularly mindful of the fact that he has in hand images of the killer, who’s white, but none of the victim, who’s black) and he heads to Dothan, which he hasn’t visited in twenty years, to investigate the story.

Another apt title for the movie could be borrowed from Rithy Panh’s documentary “The Missing Picture,” from 2013. There, Panh, a survivor of the depredations of the Khmer Rouge in Cambodia in the nineteen-seventies, uses small figurines to create new images of massacres and slave-labor camps, in lieu of photographs of these events, which do not exist. Images—the images that get made and seen and that survive, and the ones that don’t—are at the heart of “Did You Wonder Who Fired the Gun?” Wilkerson, looking at the role of a white person taking action against Southern racism, begins the movie with a brief consideration of Gregory Peck’s performance as the lawyer Atticus Finch, “a saint,” in the 1962 film of “To Kill a Mockingbird”—and concludes with a discussion of corresponding images that don’t exist, of Harper Lee’s earlier version of the book, “Go Set a Watchman,” in which Atticus is an utterly unsaintly racist.

Wilkerson makes “Did You Wonder Who Fired the Gun?” not to make images but to search for them. That search involves multiple levels of mystery, including in the effort to discover the facts surrounding the killing of Bill Spann by S. E. Branch, and in the effort to find images that bear physical traces of the victim himself. Many of the most important images in “Did You Wonder Who Fired the Gun?” are black screens—Wilkerson blanks out the image only briefly, but notably—because the images that Wilkerson finds are only substitutes, and inadequate ones, for those that don’t exist. The ultimate substitution involves Wilkerson’s search for Spann’s grave. It’s a search that takes him to the nearby town of Louisville, Alabama, where a white town clerk claims that there’s no record of the burials of black people from that era but a black official covertly, and, it seems, fearfully, takes Wilkerson aside and offers to help. Wilkerson has no images from within the squat, streetside town hall or of the helpful official—who, moreover, insisted on anonymity. But he does have images of danger from a nearby town with a reported Ku Klux Klan presence, where, as he says, he only “wanted to film trees”—and over an image of those trees he superimposes a shot, running backward, of Billie Holiday singing what is, in all likelihood, “Strange Fruit.”

Wilkerson contrasts his images of Branch’s inscribed headstone and well-tended grave with his images of the place that is somewhere near what is likely to be Spann’s unmarked burial site. And he contrasts his own status, as a white maker of those images, with the inability to trace Spann’s surviving family, even with the help of a private investigator—and defines those contrasts as the essence of racism. Images themselves—their existence, their nonexistence, their implications, their use—are more than the subject of “Did You Wonder Who Fired the Gun?”; they’re its political focus, its touchstone of conscience. As the movie advances, Wilkerson fragments the image, multiplies the image, tints the image blood-red, suggesting—as his investigation goes further and his discoveries about Branch grow more horrific—that images fail completely, that images depicting the ambient horrors of modern life are, at best, loose ends that, when pulled, make society, family, memory, and experience come apart at the seams.

At the beginning of the film, Wilkerson declares on the soundtrack that his film isn’t a “white savior” story but a “white nightmare.” By the movie’s end, the ambiguity of that idea becomes clear: the nightmare is the vision of horror that’s implied by living as a white person in a white-dominated society. One of Wilkerson’s aunts, whom he calls a “politically active white supremacist” and who is a pro-Confederacy activist, is a presence in the film; Wilkerson films a rally of the pro-Confederacy group the League of the South, in which she’s involved, and he shows a clip from a pro-Confederacy speaker. Such groups and such activities may have seemed oddly fringe when Wilkerson began work on the film, in 2013; today they have a shocking prominence and a shocking place in the center of American power.

The movie is also filled with nonexistent images of sexualized violence; Wilkerson visits the nearby town of Abbeville, the site of the rape of Recy Taylor, a black woman, by six white men, in 1944. (Rosa Parks later visited the town, working as an N.A.A.C.P. investigator, to seek justice for Taylor.) Wilkerson also learns, from his mother and from one of his aunts, that Branch wasn’t only a racist murderer. (Evidence emerges that he killed another black man in addition to Spann, also with impunity.) Branch also assaulted his wife; he was also a sexual predator, a child molester. The long-standing silence regarding these events—and the impossibility of images—is part of the story, as well.

The title of the film comes from the 1963 song “William Moore,” by Phil Ochs; Moore, a white civil-rights activist, was murdered that year in Attalla, Alabama. Wilkerson drives there and looks for the site of his shooting; it, too, is unmarked and uncertain. The lines from the song’s final verse, which Wilkerson includes in the soundtrack, are: “Did you wonder who had fired the gun? Did you know that it was you who fired the gun?” The movie’s “white nightmare” is the story of every white family—it’s the so-called American Dream, the illusory images that substitute for the nonexistent ones of American life itself, and the subject of the movie isn’t a matter of being “woke” but of simply waking up.