Photographer Ernest Withers outside the Lorraine Motel in Memphis, where the Rev. Martin Luther King Jr. was assassinated. (Thomas S. England/LIFE Images Collection/Getty Images)

Aram Goudsouzian is the author of “Down to the Crossroads: Civil Rights, Black Power, and the Meredith March Against Fear” and the co-editor, with Charles McKinney, of “An Unseen Light: Struggles for Black Freedom in Memphis, Tennessee.”

You know the photograph. It is from 1968, one week before the assassination of Martin Luther King Jr. A sea of signs, proclaiming “I AM A MAN.” A sharp line of black sanitation workers, ready to march through Memphis. A purse-wielding woman in the left foreground, and a slim man walking across the line, gazing right at the camera.

The man behind the camera was Ernest Withers. Besides that iconic photograph, he supplied scores of images that shape our memory of the civil rights movement. He captured the dramatic moment in a Mississippi courtroom when Moses Wright identified the abductor of his great-nephew Emmett Till. He snapped the perfect shot of King staring balefully out a window while integrating a bus in Montgomery. He photographed the Freedom Rides, the funeral of Medgar Evers and James Meredith’s March Against Fear.

Withers was also a paid informant for the FBI. That news broke in September 2010, after the Memphis Commercial Appeal published a report by Marc Perrusquia. The reporter had exploited a clerical error to learn that Withers had supplied information to FBI agents from at least 1968 to 1970.

“A Spy in Canaan,” by Marc Perrusquia (Melville House)

That bombshell raised as many questions as it answered. What was Withers’s exact role? When did it begin and end? What was its impact? Most important, why? Why did this great black photographer spy on a movement for black freedom?

Perrusquia’s new book, “A Spy in Canaan,” fleshes out critical details in the Withers saga. It is a triumph of investigative reporting, the product of the author’s dogged research and a bold lawsuit backed by the Commercial Appeal. It also stirs an appetite for a richer history of the civil rights movement, though it cannot satisfy that hunger.

Withers grew up in the segregated North Memphis neighborhood and served in the Pacific theater during World War II. In 1948, after joining the first crop of black police officers in Memphis, he lost his job in a bootlegging scandal. Withers turned back to photography, a passion since childhood, and thrived as a chronicler of black life in Memphis: portraits, community events, the stars of baseball’s Negro Leagues, the bluesmen of Beale Street.

A gregarious man with an entrepreneurial knack, Withers also freelanced for black publications, photographing the dawn of the black revolution. Thanks to his camera and easy smile — as well as his black skin — he entered the inner circles of civil rights leaders. It made him the perfect informant.

The FBI cultivated him as early as 1958. By the 1960s Withers was relaying to the agency photographs and gossip about the Nation of Islam in Memphis, voting rights battles in rural Fayette and Haywood counties, antiwar hippies, and nonviolent ministers such as the Rev. James Lawson. He monitored the swirl of activists and politicians coming through Memphis, a crossroads city in the black freedom struggle. In 1967 he earned the designation of confidential informant on racial matters.

His main contact, William H. Lawrence, personified the politics of the FBI. The agent suspected communist influence in any left-leaning activism. He took a “vacuum cleaner approach” to citizen surveillance and fed compromising information to local authorities, while evincing a moral distaste for black leaders such as King. Withers took his family portrait.

Perrusquia frames the book as “a narrative of discovery,” often reverting to first-person voice to describe his bumpy journey toward the truth. His first tip came in 1997, during an interview with a retired FBI agent. Eleven years later, after Withers died, a Freedom of Information Act request produced FBI files on a late-1970s prisoner pardon scandal that felled Tennessee governor Ray Blanton. Withers, then working for the state liquor board, had played a bit role. One page of the file failed to redact his code name as an informant. Perrusquia had proof.

After publishing his breakthrough story in 2010, the Commercial Appeal sued the FBI, despite the newspaper’s struggling finances. The FBI ultimately released 70 of about 150 investigative files involving the photographer. As Perrusquia writes, these files “establish Withers as a valued, long-term asset for the FBI inside the civil rights movement.”

The tragedy of 1968 hangs over the book. During the sanitation workers’ strike, the FBI was helping to shape the public discourse in Memphis. It painted the Invaders, a local Black Power organization, as a hostile threat. It portrayed King as a rabble-rouser who consorted with militants in expensive hotels. Withers supplied the authorities with key details while also taking the most resonant photos of the protesters. After the April 4 assassination, he even photographed King in the morgue.

Withers kept supplying intelligence to the FBI until 1975, by which time he had been paid more than $20,000. “A Spy in Canaan” brims with new details about the inner workings of the movement in Memphis and beyond. It rarely steps back to assess the complicated nature of black activism in this era, however.

Perrusquia instead details the nature and impact of federal surveillance of American citizens exercising their right to lawful protest. “In the name of national security,” he writes, “the FBI retarded the movement.” Its hysteria over radicalism tainted a democratic movement.

Without a doubt, Withers betrayed the activists who trusted and befriended him. Why? The money surely helped provide for his eight children. Withers also seemed uneasy with the street-theater confrontations of the civil rights movement. Maybe, too, he wanted to be a lawman again. But we do not know. The FBI never released Withers’s official informant file. Without that file, Perrusquia cannot explain the motivations of his subject.

That void has implications beyond the life of Withers. Whatever our political leanings, we tend to see the civil rights movement as a clash of heroes and villains. Withers’s photography made him an ideal FBI informant, but his FBI work also enhanced his civil rights photography. Those images left an indelible imprint on our collective understanding of an epic struggle for human justice.

Perhaps, then, our sinners are also our saints.

A Spy in Canaan
How the FBI Used a Famous Photographer to Infiltrate the Civil Rights Movement

By Marc Perrusquia

Melville House. 349 pp. $28.99