An in-depth investigation into America's oil pipelines and the flawed regulations that govern them has earned three reporters for InsideClimate News the coveted Pulitzer Prize for National Reporting.

This is only the third time that an exclusively online publication such as InsideClimate News has won the Pulitzer. The publication, which is only five years old, is also a non-profit funded by several large foundations, which further sets it apart from traditional commercial print publishers.

The award was granted to reporters Lisa Song, Elizabeth McGowan and David Hasemyer for their work on a massive story entitled "The Dilbit Disaster: Inside the Biggest Oil Spill You've Never Heard Of." The story, also available as an e-book, started as an investigation into a spill of Canadian tar sands (aka diluted bitumen, or "dilbit") into Michigan's Kalamazoo River in 2010. Their initial seven-month investigation then broadened to cover pipeline safety issues throughout the country and how America is ill-prepared to handle similar disasters that might be caused by tar sands oil, which is more corrosive then normal oil.

The Pulitzer committee cited the trio "for their rigorous reports on flawed regulation of the nation's oil pipelines, focusing on potential ecological dangers posed by diluted bitumen." The authors will share a $10,000 prize. Song is a staff reporter at InsideClimate, while McGowan recently left the organization to write a book. Hasemyer is a freelance writer who regularly contributes to the site.

InsideClimateNews founder and publisher David Sassoon said he was proud of his team and gratified by the award. "It's a watershed moment for our non-profit news organization, a good day for environmental journalism, and a hopeful signal for the future of our profession."

Susan White, the site's executive editor who was involved in every stage of the story, credited the three reporters for their hard work. "Elizabeth, Lisa and Dave believed deeply in these stories and were determined to do everything they could to make them clear and accessible to our readers. Elizabeth's ability to persuade people to talk, Lisa's science background and Dave's doggedness made it all work."

Sassoon told The Washington Post that InsideClimate News sets itself apart by actively pursuing investigative journalism, something that many news organizations have abandoned. "We try to fill in the gaps that exist in American journalism that are more and more common," he said. Pulitzer Prize administrator Sig Gissler told the Associated Press that the award to the online news site showed that this type of watchdog journalism remains strong. "It's a very hopeful sign. It really shows the way the journalism ethos reconfigures itself as times change."

The Pulitzer Prizes include awards in 14 journalism categories. There were 1,081 entries this year, with finalists representing 25 news organizations, only two of which publish online-only. Online news organizations have only been eligible for Pulitzers since 2009.