SHE scoured the streets day and night, venturing into strange and sometimes dicey neighborhoods. She wore a hat, sturdy shoes and a camera, always a camera, around her neck and at the ready.

A woman in a fur stole and evening dress drifting in the darkness toward a 1956 two-tone Chevrolet.

Click.

A curious little boy, undaunted by his size, using an empty window frame as a ladder so he can peek into a giant box.

Click.

A huddled coil of a man defeated by life, his clothes soiled and tattered, his head hanging in despair.

Click.

Fractions of seconds, captured by Vivian Maier a half-century ago or more - fleeting moments of life on the streets at a time when men wore fedoras and dragged on Lucky Strike cigarettes, when women favored babushkas, when families piled in Studebakers and Packards for Sunday drives.

Maier observed it all without judgment. This was her hobby, not her job. But over five decades, it also was her life. She shot tens of thousands of photos. Most were never printed. Many were not even developed. Few were seen by anyone but her.

Vivian Maier wanted it that way. She guarded her privacy so zealously that she did not even want people to know her full name.

She and her photos seemed destined for obscurity until a young man stopped by an auction house one day. He paid about US$400 for a grocery box stuffed with tens of thousands of negatives.

He knew only that they came from a repossessed storage locker once rented by an elderly woman.

He wasn't expecting much from them - maybe just some illustrations for a history book he was co-authoring about his neighborhood. He didn't find any.

But he did unearth a far bigger treasure.

John Maloof had stumbled upon an undiscovered artist whose photography is now being compared to the giants, a reclusive woman who, in death, is attracting the kind of attention and acclaim she would have shunned in life.

Maloof, a real estate agent, knew nothing about photography, but when scanning some of the negatives in his computer, even a novice could see they were special:

Striking scenes of every crane and every beam as Chicago's John Hancock skyscraper went up. Captivating cityscapes of the elevated tracks in New York.

Maloof was inspired to shoot his own photos. He wanted to meet Maier, but the auction house said she was ill. And he didn't press.

Instead, Maloof decided to collect as much of her work as he could find. He contacted folks who had bought Maier's other possessions at the auction that day in late 2007. Soon, he owned 1,000 rolls of her film. But it would be expensive developing them all.

So Maloof, who sells antique reproduction hardware online, peddled about 100 negatives on eBay to raise cash.

One buyer was Allan Sekula, a prominent photographer, critic and teacher. He offered some advice: Stop selling the negatives. The work was good enough for an exhibition and should not be dispersed.

Maloof set out to learn more about Vivian Maier. His first Google searches had fizzled, but in April 2009, he spotted her name scrawled on the envelope of a roll of developed film. He tried again.

This time, he found an obituary in the Chicago Tribune.

Maier had died only days earlier.

"Vivian Maier, proud native of France and Chicago resident for the last 50 years died peacefully ... A free and kindred spirit who magically touched the lives of all who knew her ... Movie critic and photographer extraordinaire ..."

Her 83 years on earth, summed up in 96 words. But one sentence stood out: "Second mother to John, Lane and Matthew." Perhaps she was their stepmother?

Maloof called the Tribune, but soon ran into some dead ends.

Then a serendipitous moment: As he was filing loose negatives he spotted an address on a box.

The address led Maloof to Lane and Matthew Gensburg, two of the brothers who had posted the obituary.

And so the mystery of Vivian Maier's life began to unravel.

Vivian Maier, it turned out, had two distinct identities: A nanny for the Gensburgs and other families in a 40-year career on Chicago's affluent North Shore. And a photographer who chronicled the gritty drama and tender moments of street life in and around Chicago.

Maloof, now 29, and partner, Anthony Rydzon, tracked down families Maier had worked for, hired genealogists and chronicled her path from New York to France to the Chicago area.

A picture of Vivian Maier slowly emerged: Fiercely independent. Eccentric. Private, yet confrontational.

She had opinions about everything: American Indians? They had gotten a raw deal. Women? Just as capable as men. Marriage? No thanks.

"She was a free spirit - that's the only way you can really describe her," Lane Gensburg says. "She had no real interest in material possessions."

As a nanny, Maier tended to move often, but she remained with the Gensburgs more than 15 years, becoming a beloved family member and staying in touch long after she left.

The three Gensburg sons, now in their 50s, helped Maloof by opening a storage locker packed with Maier's trunks, clothes, negatives, 8mm films, audio tapes - and a birth certificate showing she was born in 1926 in New York (not in France, as the obituary said) to a French mother and Austrian father.

There is no indication Vivian Maier ever studied photography.

But a genealogist located a 1930 census showing Maier and her mother once lived with an acclaimed female portrait photographer, Jeanne Bertrand. Maier was only a child, but Maloof says that environment may have provided inspiration. Or a start.

Maier's photos range from 1949 to the mid-1990s. Mostly, her black-and-white images depict the poor, women (especially well-dressed dowagers) and children. After switching to color in the mid to late-1970s, she turned to graffiti, inanimate objects and garbage.

Maier also shot self-portraits, her face seen through a window pane or some other reflection.

So what makes her work special?

"She had an open and inclusive and very fundamental idea of what constituted 'America' that was missed by a lot of photographers in the 1950s and 1960s," says Allan Sekula, who bought some of her negatives and teaches at the California Institute of the Arts.

"It's both the variety and a kind of quirky democratic energy of street life," he says. "She connected herself through the camera to the street in a way that gave her a charmed presence."

Maier made 3,000 to 4,000 prints. Maloof owns about 100,000 negatives and so far, has scanned only about 10 percent of them. (Another collector has about 12,000). About a third of Maier's work remains in the rolls.

Shortly after Maloof started buying Maier's film, he tried, to no avail, to interest galleries and museums. Then he posted a blog he had created featuring her work on Flickr, the online photo-sharing site, and asked what he should do with the negatives.

Ideas poured in.

This winter, Maloof finally succeeded in getting a one-woman show for Maier at the Chicago Cultural Center, her US debut.

Maloof is now working with Sekula to assemble a book of her photos. He also has joined his partner, Rydzon, and a Danish filmmaker to produce a documentary, "Finding Vivian Maier."

He is not, he insists, looking to make money on her legacy. He says he merely wants to bring her photos to the world, and bring credit to an unsung artist.

Would the intensely private Maier approve?

"She wouldn't like all this attention, but I feel her work deserves recognition, and I think this is a nice thing for her legacy," Maloof says. "But I'll never know, of course, what she would think."