Vivian Maier was a mysterious character. The nanny-photographer left behind thousands of photographs, but little written information about her life.

That’s why I flew to France: to find more about Ms. Maier’s French roots.

Her negatives were found in an abandoned storage locker in Chicago in 2007 and sold at auction later that year. By the time the buyers realized — some two years later — exactly what they had, Ms. Maier had passed away at age 83. Since the first of her photographs were posted online, people from all over the world have marveled at the humanity of the work. Just recently, a friend of mine met a street vendor in Paris and told him she was working on the Vivian Maier archive.

He grew silent for a moment, before he said: “Vivian Maier makes me cry.” Then he gave her a sandwich for free.

Like any biographer, I am determined to travel in the footsteps of my subjects. For Ms. Maier, that means a trip to the high Alps — to a tiny town called Saint-Bonnet-en-Champsaur about five hours by car north and west of Nice on the French Riviera. I knew that Ms. Maier spent some of her childhood in this and other tiny towns surrounding Saint-Bonnet, but I had few details.

Michael Williams and I are writing a book, “Vivian Maier: Out of the Shadows,” which will be published by CityFiles Press. It’s the first book that presents Ms. Maier’s photographs in the context of her life story. Based on a collection of about 18,000 Maier negatives owned by Jeffrey Goldstein, the book is due out in September.

We have managed to track down people who knew Ms. Maier during the last 50 years of her life. But her years in France remained something of a mystery.

She was born in the Bronx in 1926, and moved to the tiny town of Saint-Bonnet in 1932. Her mother, Maria, was born nearby but moved to New York in 1914. She married Charles Maier in 1919, but they separated in the late 1920s. During the height of the Great Depression, Maria returned to France with her baby daughter. They stayed six years before coming back to America. Vivian was drawn back to the Alps in 1949 and again in 1959 to photograph the land she loved.

Before Jeffrey and I left for France, we made contact with three people — Philippe Escallier, Jean-Marie Millon and Monique Escallier — who have doggedly researched Ms. Maier’s roots on a volunteer basis since they first learned about her a few years ago. They have combed administrative records to find records on Ms. Maier and her family, figured out her maternal family tree (her father was from Austro-Hungary) going back centuries and used Google Maps to pinpoint what she photographed.

Jeffrey and I never expected what was waiting for us after we met Philippe, who also served as our translator. We greeted each other at a tollway station just outside the city of Gap at the southern end of the Champsaur valley, and were immediately taken to Le Dauphiné Libéré, a regional newspaper that was doing a story about us. Despite the fact that we were only there to find out what the French knew about Ms. Maier’s early life, we were welcomed as if we were triumphant G.I.’s from Normandy. The town takes great pride in its connection to Ms. Maier and has long had a strong bond with the United States.

Next, we were taken to a nearby playground Ms. Maier photographed in 1949. There, we met about 20 residents, in their late 60s and early 70s, who gathered to see the photos she took decades earlier of children at play from the Rochasson neighborhood of Gap. Amazingly, most of those who came to the playground that day identified themselves as children in the photos. They found neighbors and friends — some alive and some dead — as well.

I didn’t see any tears, but I could feel the emotion and hear a continuous stream of chatter as they huddled together. The photos brought back memories of a time when children came home for lunch with their mothers, and when U.S. soldiers stationed in Gap introduced the children to chewing gum. All that is long gone, they told us.

Not one of the adults who studied the photographs remembered Ms. Maier. They were far too preoccupied as children to have noticed her. But the pictures meant a great deal to them since few had ever seen any such photos. The late 1940s was an austere time in post-World War II France. A few of the adults had formal classroom pictures of themselves, but none had any causal photos of that time because few people in Gap walked around with cameras. They smiled with glee.

We admire Ms. Maier’s work because it tugs at our emotions and seems so universal, but as I watched the French townspeople connect to these photos, I understood that another value of her 100,000 photographs is on strictly a documentary level. Cameras are time machines. Those who embrace them help us all travel back.

The residents on the playground didn’t want to leave. They applauded Jeffrey when he announced (with Philippe translating) that he was leaving a set of photos on a computer hard drive there in Gap. “They belong to you,” he told them. They looked excited — and stunned.

We spent the next two days in the heart of Vivianland, squired around by our three Vivianphiles. We drove up the mountains she loved to climb. We saw the apartments where she lived, the small estate she inherited and the photography store where she developed her earliest work.

What was most surprising was how many people our hosts tracked down who knew Ms. Maier in the 1930s. Now in their 80s and 90s, they described young Vivian as the mysterious girl from New York who came to the Champsaur knowing no French. They remembered playing games with her, visiting her apartment, and they recalled in detail her return in 1949 and again in ’59. She did learn French, but with an accent, they said. Then, after her last visit, they lost touch with her.

Raymond Pascal, who lived only a few doors from Ms. Maier in the 1930s and described her as a “very beautiful girl,” said he visited Chicago not long ago.

“If only I knew,” he said.

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On Thursday, “This American Life” will present a live show onstage in New York. The program, which will also be shown in movie theaters throughout North America, will feature a segment on Ms. Maier’s life.