It's the biggest mystery in the world of photography. When amateur historian John Maloof bid on a box of old photo negatives at an estate auction in 2007, he had no idea what he was getting himself into. Once he scanned some of the images inside, he was intrigued to find a cache of incredible street photographs mostly dating from the 1950s and 1960s.

The shooter was Vivian Maier (1926-2009), an artist who is now mentioned in the same breath as Robert Frank and Diane Arbus. But at the time of Maloof's discovery seven years ago, she was unknown.

Finding Vivian Maier chronicles Maloof's efforts as a custodian for Maier's work and his attempts to unravel the mysteries around the 20th century's most enigmatic photographer. Who was this woman? Why did she take so many photographs? Why did she so seldom develop the films?

Maier, we learn from her various former charges, worked as a nanny for most of her life for such well-heeled Chicagoans as talk show host Phil Donahue. From behind her twin lens Rollieflex, she was frugal with her film stock but generous with her focus. Her pictures could be humorous, tragic, bittersweet, candid or puffed up.

Maier is as frustratingly hard to pin down as her style. She was born in New York but spoke with a French accent, a detail that leads to a lovely piece of detective work in this fascinating documentary. An elusive contrarian, she had a talent for postmodern reinvention long before such terms were popular currency. She might be Viv or Vivian or Maier or Mayer. Looking at her many, cleverly staged self-portraits, there’s a sense of performance for the camera.

Maloof and his co-director Charlie Siskel have worked hard to document Vivian as she documented others. But years after her anonymous death, she remains one step ahead of would-be investigators. Finding Vivian Maier is not without its frustrations. And that, we suspect, is exactly what she would have wanted.