After word broke earlier this week that The Huffington Post was looking to raise a new round to fund “investigative journalism,” the paper work on that round was actually being signed, and it’s a nice $15 million.

The Times broke the story and notes that the money will be used to expand The Huffington Post’s move into local journalism as well as investigative reporting. The Huffpost launched a local site for Chicago earlier this year. Paid Content claims the new round came from Oak Investment Partners. The site has previously raised $12 million over 3 rounds. Valuation on the new round was $100 million.

The key for the Huffington Post is to keep traffic up post election. Quantcast shows rapid growth to a staggering 302 million page views on 12 million people monthly to today, but the same stats also show the start of a steady decline down after election day.

The Huffington Post has always been an interesting mix of ideas: the site has grown its full time staff (around 20-30 people as of a few months ago) but most contributors to the site do so for free, so its part traditional news site, part blog and part Citizen Journalism play.