In "The Untold Story of Emmett Louis Till," about the notorious lynching in Mississippi in 1955, we see how a grieving mother created a landmark moment in American history.

The sheriff in the Delta county where Till was murdered 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. But then the call came from Chicago: Mamie Till Mobley, the 14-year-old's mother, 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. Her son had been shot through the head, then tied by the neck with barbed wire to a heavy fan and dumped in the river. This, for supposedly whistling at a white woman at a country store.

The two killers were acquitted by an all-white jury in nothing flat, which only raised the nation's fear and loathing of the white South, and the moral impetus for the civil rights movement was crystallized.

This is powerful, and often-told, material. But for a documentary that bills itself as "untold," director Kevin Beauchamp never makes clear what he's telling that is stunningly new. Apparently, the new information is in some interviews in which some new possible accomplices are named. But there is no "gee-whiz" moment of revelation and no helpful narration to tell us what's new and what's simply a fresh retelling.

You leave the theater feeling moved by a mother's courage, sickened by the crime and a little frustrated, wondering whether this unquiet moment in our history -- the Till murder case has been reopened -- will ever rest easy.

-- Neely Tucker

A documentary recounts the 1955 lynching death of Emmett Louis Till, 14, with mother Mamie Till Mobley.