The FBI mole at Dr Martin Luther King's right hand

A photographer who captured intimate images of iconic moments in the history of the US civil rights struggle has been revealed as an informant for the FBI. As the undeclared doyenne of black press photographers in the 1960s in the American South, Ernest Withers, who had a shop front in Memphis, Tennessee, was able to photograph Dr Martin Luther King and the innermost circles of the leadership.

When King was assassinated in 1968 after arriving in Memphis to support a strike by the city cleaning crews, it was Withers who found himself inside the Lorraine Motel photographing the blood on the floor.

He first got notice as the photographer present through the 1955 trial of the killers of Emmett Till, murdered in Mississippi for allegedly whistling at a white woman. It was Till's death that arguably first ignited the civil rights movement. That image of King riding one of the first desegregated buses in Montgomery, Alabama – Withers took it.

Withers, who died aged 85 in 2007, became known as the "original civil rights photographer". His family was recently exploring plans to open a museum in his name. But the respect and the affection he earned risks now being tainted. The FBI, led then by J Edgar Hoover, had another moniker for him – a "racial informant".

Details of his alleged moonlighting as an FBI mole were published by The Commercial Appeal, the main Memphis newspaper, after what it said had been a two-year investigation. It acquired FBI reports of its dealings with an agent called ME 338-R after lodging a request under the Freedom of Information Act. Apparent clerical mistakes committed by the FBI at the time meant that the paper could confirm that ME 338-R was in fact Withers.

The papers showed that, from 1968 to 1970 at least, Withers was passing on assorted nuggets about top activists who had given him their trust to FBI agents in Memphis, including everything from details about plans for upcoming marches, the political utterances of leaders like King and even licence-plate numbers. His usefulness was noted in one report that described him as "most conversant with all key activities in the Negro community".

A daughter of the photographer told the newspaper that its investigation was not conclusive. Rosalind Withers said, "My father's not here to defend himself. That is a very, very strong, strong accusation."

The reports are as much a surprise to surviving activists from the era as it is for the family. "If this is true, then Ernie abused our friendship," said the Rev James Lawson, a retired minister who organised civil rights rallies throughout the South in the 1960s.

Joseph Lowery, who was a close confidant of King, agreed that it would "have been an act of betrayal". But he added that we "never had any information that was sensitive. We never did anything that was covert. We didn't plan ambushes... He was just playing the FBI for a sucker. What was he going to give them that would be useful?"

Andrew Young, a leading figure of the movement who later became Mayor of Atlanta, also cautioned against over-reacting, saying that there was little to hide from the FBI anyway. "I always liked him because he was a good photographer. I don't think Dr King would have minded him making a little money on the side," he told The Commercial Appeal.

Nor was it any secret to anyone that the FBI was snooping, and more. A programme called Cointelpro launched in the 1950s initially to trail and also, if possible, discredit members of the Communist Party, had been shifting its focus to civil rights activists through the 1960s. It was under its umbrella that the FBI pursued its campaign not just to keep tabs on King but to discredit him.

But the extent of Withers' co-operation with the FBI may nonetheless be surprising. It seems he even passed on details of conversations between activists at King's funeral about what support should still be given to the striking Memphis workers.

"It's something you would expect in the most ruthless, totalitarian regimes," said D'Army Bailey, a retired judge and former activist who was himself watched by the FBI.