THIRTY YEARS ago this Tuesday, America discovered its collective conscience. In a photograph published on front pages virtually everywhere but in its city of origin, a German shepherd held by a tooth-baring cop attacked a black youth, apparently one of nearly 3,000 school-age freedom marchers recruited by Martin Luther King Jr., for the nonviolent demonstrations that would consecrate 1963 in civil rights history as "The Year of Birmingham." "The image of the savage attack struck like lightning in the American mind," as Taylor Branch wrote. It made President Kennedy sick. Through the picture's timeless window, Americans saw the Nazi storm troopers against the Jews or the more distant archetype of bloodhounds chasing Eliza across the ice floes.

If textbooks listed "Uncle Tom's Cabin" as a major impetus of the Civil War, so did this photograph become one of those rare cultural artifacts that change history. It nationalized the African-American freedom movement and led to the first serious civil rights legislation since Reconstruction, eventually passed as the Civil Rights Act of 1964, abolishing legal segregation.

The photograph magnificently captured the Big Truth about segregation, evil in black and white. Yet in most particulars, it was as much a fiction as Harriet Beecher Stowe's novel. The actual events of May 3 were tinged with the ironic grays of most great conflicts. In the complex dynamics of lib-eration, nonviolence courted violence, suffering courted show biz and the segregationist "system" courted the inadvertent heroism of an individual life.

For one thing, the police dogs -- and their demonological other half, the firehoses blasted at the demonstrators -- were hardly unwelcome to the strategists of the Southern Christian Leadership Conference's critical campaign. Birmingham had been targeted precisely because SCLC needed the picturesque brutality of Commissioner Eugene "Bull" Connor to dramatize black oppression.

"We've got a movement," SCLC's chief tactician, Wyatt Tee Walker, had reportedly rejoiced when the K-9 Corps made a brief appearance five days into the campaign, on April 7. But the press laid the violence that day to black bystanders, and after a month of dwindling marches, national sympathy had still not graced the protest. Desperate for troops, SCLC sent children by the hundreds into the streets on May 2.

Fifteen-year-old Walter Gadsden was not one of them. The one-time honor student, member of a black middle class that resented King's intervention, had been playing hooky even before the demonstrations made truancy honorable -- "running with a bad crowd," he would tell Jet magazine later that year. On May 3, he was merely watching his nonviolence-disciplined classmates. Unfazed by dogs (he had a large one at home) he was crossing the street when he stumbled into the martyrdom that would make civil rights the Kennedys' urgent priority.

Officer R.E. "Dick" Middleton, the son of a U.S. Steel crane operator with a secret passion for cooking, was known among colleagues for being quiet and reserved, typical of the straight arrows that the K-9 Corps preferred over the department's race ideologues. As for the dogs, they were already celebrities. The top canine, Rebel, performed before civic groups and school kids all over town.

When Officer Middleton's charge, Leo, lunged at Walter Gadsden's ribs, the youth reflexively thrust his knee into the dog's chest. But in photographic reality, his bent knee, with his slightly stooped body, conveyed classic nonviolent acquiescence, and his bowed head registered the composure of a saint.

During the short half hour that the dogs were in action, never to reappear, they furnished the coveted images of savagery, while inflicting almost none. In an example of poetic justice not lost on the movement, a few dog bites had mustered the moral suasion that a post-slavery century of psychic and bodily terrorism could not. "There was no 'Battle of Ingram Park,' " Wyatt Walker would say of the scene, in which participants waved their shirts at the dogs like matadors and shimmied their backsides at the hose spray. "It was a Roman holiday."

To be sure, many witnesses, including a battle-jaded press corps, did not share Walker's retrospective breeziness. But a jolly countermythology spread through the local black grapevine: false rumors that Walter Gadsden's knee had broken Leo's jaw, that dogs got away from their handlers and were cut to death. This illusion of bloodied German shepherds was an important psychological symptom of emancipation: blacks were losing their fear of the oppressor. Indeed, inspection of the famous photograph reveals a dazzling effrontery, Gadsden's left hand grasping Middleton's wrist.

Over the years, the players in this watershed have remained anonymous -- not the least among them the photographer, then a 31-year-old Associated Press man based in Memphis, whose priority that day was "staying alive." For his trouble William Hudson got no fame (the photo credit went to AP), no money ("not a nickel" bonus), no glory. Now back in his hometown of Miami, Hudson does portraits of another common domestic species, cats.

The dog Hudson immortalized died of old age. His handler, now retired, operates a fine German bakery in a Birmingham suburb. (Middleton had a German wife as well as a German shepherd.) He refuses to discuss the events of May 3, except to say that the picture "didn't bother me."

For nearly a decade I have tried to find Walter Gadsden. Thirty years ago he told Jet that his encounter with Leo had inspired him to study and prepare for college. I wondered what happened to this icon of the history that I, also a child of Birmingham 1963, would grow up to write.

On a hunch one recent evening, I found myself dialing directory assistance for a city I won't name. The operator gave me Gadsden's number. I dialed it, anticipating a hometown reunion of sorts. With a cold apprehension, Walter Gadsden told me he did not want to "become involved" in my story and politely hung up.

Diane McWhorter is writing a book about Birmingham.