When Chicago real estate agent John Maloof bought an unmarked box of some 40,000 negatives at an auction house in 2007 for $380, he had no idea what he had stumbled upon. It was a body of undiscovered work by Vivian Maier, who has posthumously been recognized as one of 20th-century America's great street photographers.

Maier was a French-American nanny born in 1926, who shot pictures constantly, but never exhibited them. "At first I didn't know that this was great street photography," Maloof says in a phone interview.

The Maier images he posted online and the ones which have now been exhibited around the world and collected in a handful of books are "a tiny, tiny fraction of her work," Maloof says. "Every photographer has an overwhelming amount of horrible work, so you have to go through a lot of negatives. Even if you pick up one and it's great, there will be a ton that are not going to be great."

Nevertheless, Maloof says that, as he unearthed more of Maier's best work, he was inspired both to start shooting pictures himself and learn more about photography.

In 2008 and 2009, he spent $70,000 or so buying what he estimates amounts to 90 percent of Maier's body of work from others who had purchased boxes at the auction house. He is now the custodian of some 100,000 negatives.

Ever since his first purchase, Maloof has been trying to unravel the mystery of Maier, a woman who spent nearly half a century taking photos that she never shared with the world.

After he discovered Maier's name in one of the boxes, a Google search for information on her yielded no results -- except for her obituary in 2009.

Maloof eventually found an address that gave him a lead to people she had worked for as a nanny in Chicago. When he met them, they told him they had a storage locker full of her stuff, which was headed for the dumpster.

He took the whole lot -- newspapers, receipts with varying spellings of her name, books, clothes, hats, letters and more -- to add to the mountain of negatives, the hundreds of hours of Super 8 video and audio recordings and cameras she had left behind.

"It was overwhelming," Maloof says. "There's no way you can go through it all. She saved everything. Every single book had a note in it or a coupon or a receipt. She was such a hoarder. I mean really."

So much stuff, but so few answers about the mysterious nanny. Talking with people who had been in contact with her further stoked Maloof's curiosity. He joined forces with filmmaker Charlie Siskel to make the documentary "Finding Vivian Maier," which screened late last year in New York and Los Angeles and opened April 11 in the Bay Area.

In it, the filmmakers show dozens of people -- including parents who hired her, children she cared for, store owners, movie theater operators and neighbors who remember her -- from Chicago to France.

"I wanted to figure out who she was," says Maloof. "Why (was) she taking all these pictures? What compelled her to take them, and why she didn't show the work? I just wanted to find out more about her, and each person had a different take."

The film suggests Maier was intensely private, socially awkward, estranged from her family, a loner. Even those who shared the same roof with her seemed merely to observe the eccentric Maier, who kept her bedroom door locked and fiercely guarded her boxes of worldly possessions, without ever knowing exactly who she was.

Some speculate that she wouldn't have wanted or been able to handle all the attention that has since been paid to her extraordinary work. But in one poignant scene, Maloof uncovers a letter in which Maier expressed a sadly unrealized desire to put herself out in the world.

Nobody will ever know the scope of Maier's intentions. But the compulsion to record, if not edit, her vivid images of 20th-century life has made the world a richer place.