In "The Untold Story of Emmett Louis Till," the latest documentary film about the notorious lynching in Mississippi half a century ago, we see how a grieving mother created a landmark moment in American history.

The sheriff in the Delta county where Till was murdered in August 1955 ordered the boy's mutilated corpse to be buried almost immediately after it was dredged from the Tallahatchie River, relatives recount in this fast-paced retelling. The funeral had taken place and they were about to bury the body when the call came from Chicago -- Mamie Till Mobley, the 14-year-old's mother, was having none of it.

She ordered the pine box containing her son to be sent home. She then had it pried open and displayed what was inside to all and sundry.

It was a genuinely shocking moment -- the disfigured face, an eye out of the socket, the facial skin almost detached from the bone. He had been shot through the head. Then he had been tied by the neck with barbed wire to a 70-pound fan and dumped in the river.

This, for supposedly whistling at a woman at a country store in a two-bit town called Money.

The decision to display that rotting corpse was an act of grief and courage and outrage and, in the end, sheer brilliance. Till's mutilated face exposed Mississippi racism for what it was -- a sadistic mixture of bloodlust, sexual hysteria and a level of violence worthy of a psychopath. Till's murder became myth and history personified. All the age-old word-of-mouth horror stories of what white Southern men would do to black men who dared approach white women were put on display in a Chicago church, and, famously, in photographs in Jet magazine.

The outrage Mobley created with that open-casket funeral set the stage for the most racially charged trial in the Deep South since the Scottsboro Boys. Of course, the two killers, Roy Bryant, the offended woman's husband, and his half-brother, J.W. Milam, were acquitted by an all-white jury in nothing flat. This raised the nation's fear and loathing of the white South, and the moral impetus for the civil rights movement was crystallized.

This is a familiar story, told in other documentaries, books and lengthy magazine stories, most notably in a Life magazine story in which the accused acknowledged their guilt after being acquitted, and thus were protected from double jeopardy. Director Kevin Beauchamp therefore faces a heavy burden in telling us what's new, or, as the title has it, "untold."

He sets out the story in straight chronological order, with current interviews with Till's now-elderly friends and family to recount the days before and after the killing. There is no narration, only placards to move us along to each chapter.

The best moments, by far, are the black-and-white archival footage. There is the all-white courtroom, people fanning themselves, the unusual close-ups of faces, Mobley's remarkable composure at the time. We see Medgar Evers there, eight years before his assassination. When Bryant gives his wife a long, open-mouth kiss on camera just after acquittal -- they look like they're at the twin bill at the drive-in -- it makes the flesh crawl.

The archival interviews with Moses Wright, the dead boy's grandfather, belong in a museum.

Old even then, unadorned, plain-spoken, country, he gave the trial its most electrifying moment. On the witness stand, he was asked if he saw the killer in the courtroom. He stood and pointed at the men -- this iconic image is in the film -- and uttered the immortal two-syllable indictment: "Thar he."

That is what is known as poetry, and it is frustrating the documentary doesn't have more of it. For a movie that bills itself as "untold," there's no "gee-whiz" moment of revelation. There are many fine interviews, apparently some of them new, but there is no narration to tell us which ones. There is a predictable soundtrack of tinkling piano and a gospel choir. There is a disjointed series of events at the end of the film, including what appears to be a New York City Council meeting where the killing was discussed, but I wasn't sure why it was included.

Further, there are no significant modern-day interviews with any of the white participants in the trial, the killer's families, or even local political figures discussing the case. (Both killers are now deceased. They were shunned by the white community in Mississippi after their magazine tell-all and did not lead happy lives thereafter. There's no mention of that sort of interesting postscript.) If those people refused to be interviewed, it would have been helpful to know, and a subtle buttressing of the argument that perhaps people still living know more than they are saying.

The Till murder case has been reopened -- the film tells us it's because of the film itself, an odd bit of self-reference -- but I left the theater not knowing exactly what the new evidence was. Does it tell us that other people might have been involved? Yes, but that's always been known. Are some named here? Well, sort of, but as I understood it, these were black men who may have been coerced into helping subdue Till that night.

The movie rolls to an end, the lights come up, and you leave the theater feeling moved by a mother's courage, sickened by the crime and a little frustrated, wondering if this unquiet moment in our history will ever rest easy.

The Untold Story of Emmett Louis Till (70 minutes, at Landmark's E-Street Cinema and the Magic Johnson Theater) is not rated and contains disturbing images, racially offensive terms (in older footage) and explicit discussions of violence.

Director Kevin Beauchamp purports to tell what's "untold" in the story of Emmett Louis Till's murder in the South and the actions of his mother, Mamie Till Mobley.