Vivian Maier was a mysterious and eccentric nanny who spent a lifetime looking after other people's children while harbouring a rather lovely secret: she was an astonishingly accomplished photographer.
The Guardian newspaper on Tuesday publishes rarely seen photographs by a woman now considered one of the finest street photographers of the 20th century.
A documentary film released on Friday will tell the remarkable story of Maier and the photographs she took – and then deliberately kept secret.
Maier is today considered a genius whose photographs stand comparison with names such as Diane Arbus and Robert Frank.
But if it had not been for a chance discovery at a Chicago thrift auction in 2007, the world would still be unaware of her life and talents.
The discovery was made by a young former estate agent called John Maloof who was writing a history book on his Chicago neighbourhood.
He said: "I was wondering how I would find enough old photos to illustrate the book and tried my luck at a local junk and furniture auction house."
Maloof bought a box packed with about 30,000 negatives, which he did not use in the end.
"However, I knew to keep them. I thought: 'I'm resourceful. I'll look at them later when I have more time. Fast forward two years later, that purchase had unearthed some of the finest street photography of the 20th century."
Maloof set about finding out who Maier was, and decided also to make a film documenting his discoveries.
"My obsession drove us to compile a library of interviews and strange stories from across the globe. We found roughly 100 people who had contact with Vivian Maier. In the film we let people speak for themselves.
"I hope that this story comes through honest and pure, and does more than just uncover a mysterious artist but tells a story that changed the history of photography."
Maloof has made the film with Charlie Siskel, who produced Michael Moore's film Bowling for Columbine. The executive producer is Jeff Garlin, who has many credits but will be forever famous as Larry David's agent in eight seasons of Curb Your Enthusiasm.
Maier's day job for 40 years was as a nanny working for families in Chicago, often taking her charges out with her when she was taking photographs.
Because she had no permanent home, she kept all her negatives in a Chicago storage facility. She died in 2009, too early to know about the high regard she is held in today.
Siskel acknowledged that "if Vivian Maier had her choice the world would know nothing of her life and photographs. She chose to conceal herself and her art during her lifetime.
"But hiding one's art is, of course, the opposite of destroying it. Maier preserved her work and left its fate to others."
Since the discovery of Maier's talents she has become a phenomenon, with galleries selling her prints for upwards of $2,000 (£1,200).
There have been books, exhibitions and a BBC Imagine documentary which called her "a poet of suburbia" and a "Mary Poppins with a camera".
Siskel said Maier was "a kind of spy" capturing street life and "recording humanity as it appeared, wherever it appeared – in stockyards, slums and suburbia itself".
But she was also an outsider and Siskel believed she "may have secretly longed for the family bonds she witnessed intimately for decades".
He added: "Her work is now part of the history of photography and an undeniable treasure. The discovery of Maier's work not only gave her story an ending, there would be no story without it."
Finding Vivian Maier is released on Friday 18 July.
