She tells the story simply and with amazing emotional restraint. Mamie Till-Mobley, a round-faced African American woman, sheds no tears as she describes her insistence back then that the sealed coffin be opened so that she could inspect the body of her only child.

She checked over the corpse, starting at the feet and working her way up. The midsection, she said, was intact. But the head, that was a different matter.

Much of the face had been battered and pulverized. And why was it necessary, she asked, after all of that brutality, for her son's kidnappers to put a bullet through his head?

Mrs. Till-Mobley, then Mamie Till, was examining the body of Emmett Till, a lynching victim whose body had been retrieved in the summer of 1955 from the Tallahatchie River in Mississippi. What Mrs. Till did next would ultimately shine a harsh and international spotlight on the racial practices and policies of the American South and help fuel the civil rights movement of the 1950s and '60s.

Mamie Till, at the time a widow of a military service man, had seen Emmett off that summer as he boarded a train that would take him from the family home in Chicago to the Delta country of Mississippi for a visit with relatives and friends.

Mrs. Till had given her son a distinctive ring that had belonged to his father. And as Emmett prepared for his trip south, she also gave him a warning, she recalls in "The Murder of Emmett Till," a PBS "American Experience" documentary airing Monday at 9 p.m. He would have to be careful in Mississippi, she told him. It wasn't a place that would tolerate a high-spirited 14-year-old the way Chicago had. And, she told him, if a white woman should come down the street, he should get off the sidewalk and be careful to avert his eyes.

On a hot August day, Emmett and seven friends went into a store in Money, Miss., for refreshments. One of those companions recalls in the film that there was a commotion in the store, that Till had whistled at Carolyn Bryant, the wife of the store's owner, and that she had left, perhaps to retrieve a gun. Till and his friends hurried away and hoped that the incident would blow over.

But a few days later, early on the morning of Aug. 28, a number of men brandishing guns and flashlights came to the house where Till was staying and took him from the home. Three days later Till's body was found in the Tallahatchie. It was weighted down with a 75-pound cotton-gin fan. The contraption had been tied around his neck with barbed wire. His face was hardly recognizable. But the ring he wore was unmistakable.

When Till's body was returned to Chicago, his mother not only decided that she must see the corpse for herself, she decided that everybody else should see it too.

"I don't think she had any idea then what the total reaction would be," said Stanley Nelson, the producer and director of the film. "She, as she said, wanted the world to know what had happened to her son. And she wanted people to see and feel the horror she felt. But one of the things that's remarkable was that she would have to live with it for the rest of her life. Now there would be no closure."

A viewing of the body in a South Side Chicago church drew some 50,000 people. The film shows a long line of mourners filing past the open casket, many of them growing faint or hysterical at the sight. Jet magazine, an African American publication, gave the story a ghastly national exposure when it ran a picture of Till's corpse.

On a recent visit with Mrs. Till-Mobley, Nelson said, a journalist called with questions about her son's slaying. After she hung up the phone, "She looked up and said, 'I didn't realize this would be my life for the rest of my life.' If she had let it go," said Nelson, "it would have been buried and that may have been the end of it." Instead, he said it turned into "an open wound that you're reminded of every day for the rest of your life."

The killing of Emmett Till also is an open wound, or at least a touchstone, for a generation of African American men who learned of the case as they entered their own man-child years. And, perhaps, for the parents who taught them about it. The case stood as a warning to young men of color, who perhaps had enjoyed the relative freedom of the North, that a different code was enforced in the South. And the taboo concerning black men associating with white women needed no further object lesson.

"I'm 51," said Nelson, who is African American. "I was 4 years old at the time of the case. If you had asked me, I would have guessed that the killing took place in the sixties or late fifties and that I knew about it then. It's such a part of my memory, I have no idea how I knew about it. It's simply a part of my memory."

In the 75 tears prior to the killing of Emmett Till, the film notes, about 500 African Americans had been lynched in Mississippi, most of them men accused of inappropriate attention to white women. The Till incident stood out among the others.

"The case was so stark," said Nelson. "This was a 14-year-old boy, who'd just turned 14. He was so harmless. He'd just whistled. And they came for him in the middle of the night without even covering their faces. . . . And then they got off. It was your worst nightmare of racism in America."

The film, narrated by Andre Braugher, follows the case through the arrest and prosecution of the store owner, Ray Bryant, and another white man, J. W. Milam. Their trial featured the same racism that surrounded Till's killing.

Spectators were segregated in the courtroom. Local authorities, notably the sheriff, used racial epithets with a casual air. Visiting journalists were shocked by the blatant racism, the film notes, and they wrote about it.

The Delta, noted Nelson, was a new destination for many in the national media. Previously, he said, "there was no real reason for the press to go to Mississippi. People who were well traveled had never been there. It shocked everybody."

The five-day trial included damaging testimony from two black men who left the state after appearing in court. The jury deliberated for just over an hour, and from time to time laughter was heard emanating from the jury room. Bryant and Milam were acquitted. Appeals for a federal investigation, the film notes, were turned down by President Dwight Eisenhower and FBI Director J. Edgar Hoover.

The Emmett Till case "made people realize that it was the last straw," said Nelson. "It provided an emotional spark that said this has to change, if something this dreadful can happen with no recourse. The next step is that we have to change it. The local, state and federal governments were not willing to get involved. That's what the civil rights movement was about," he said, people changing the system.

In the aftermath of the acquittal, the film notes, donations poured in to civil rights organizations. About 100 days after Emmett Till's death, Rosa Parks refused to take a back seat on a city bus and precipitated the Montgomery bus boycott. Both events stand as ignition points in the civil rights movement.

Nelson, a winner of a MacArthur Fellowship, said he was drawn to the Till story for a documentary after listening to Mrs. Till-Mobley discuss it.

"We became interested after hearing her on the radio three or four years ago and being struck at how articulate and clear she was," he said. "She was 78 then, and if the story was going to be told, it should be told soon."

The story is conveyed entirely through comments from witnesses to events of that time. There are no historians offering summations or conjecture. Some of the most trenchant observations belong to William Winter, a Democrat, who served as Mississippi's governor from 1980 to 1984. "I believe he was in the state legislature at the time" of the Till killing, said Nelson. "He was candid; he was open, he was thoughtful. White people in the Deep South have had to think about race more fully than we have in the North."

Nelson's documentary airs at a time when media interest in the Till case is being rekindled. Mrs. Till-Mobley continued to speak publicly about her son's killing until her death on Jan. 6. She was 81. Another filmmaker, Keith A. Beauchamp, is assembling a piece on the case titled "The Untold Story of Emmett Louis Till." There also is a book out, "The Lynching of Emmett Till: A Documentary Narrative."

The media buzz has raised a number of questions about details of the case. For instance, it has been mentioned that Till, a polio victim, may have been left with a speech impediment that prompted him to make a whistle-like sound when he spoke. "Emmett's cousin said Emmett whistled," said Nelson. "So we went with that story. It's really irrelevant, whether he whistled, whether he had a stutter, or touched the woman's hand. He did not deserve to die. We did not want to spend a lot of time on that."

Perhaps the largest question being posed amid the burst of media attention to the Till killing is whether the case should be reopened. Some witness accounts suggest that Bryan and Milam, who have since died, did not act alone.

At screenings of Nelson's film, which has been accepted in the Sundance Film Festival competition for documentaries, postcards are handed out, addressed to the attorney general of the state of Mississippi. The producer hopes that after seeing the film, members of the audience will fill out and mail the cards asking that the case be reopened.

The impact of the 60-minute documentary is expanded by the capabilities of the PBS Web site (www.pbs.org). There, for instance, can be found the story given to Look magazine by the two defendants in the case. After their acquittal, and with immunity from further prosecution, Bryant and Milam sold their story to the publication for $4,000.

In the article, they described in detail how they kidnapped and killed Emmett Till.