What happened to the ACORN investigation?

During a conversation with a friend of mine who did vote for Obama, she brought up something that I hadn't really thought of in some time. What ever happened to the federal investigation of Acorn for violation of laws governing their "tax exempt" status for a non partisan organization?

As this video shows, the "non partisan" organization's own chief organizer was calling for voters to vote for Obama.

http://www.youtube.com/watch?v=YfUEPeO2_pc

Now, under 501(c) Organizations described in section 501(c)(3) are prohibited from conducting political campaign activities to intervene in elections to public office. The Internal Revenue Service website elaborates upon this prohibition as follows:

"Under the Internal Revenue Code, all section 501(c) organizations are absolutely prohibited from directly or indirectly participating in, or intervening in, any political campaign on behalf of (or in opposition to) any candidate for elective public office. Contributions to political campaign funds or public statements of position (verbal or written) made on behalf of the organization in favor of or in opposition to any candidate for public office clearly violate the prohibition against political campaign activity. Violating this prohibition may result in denial or revocation of tax-exempt status and the imposition of certain excise taxes.

So, back to the question. If their actions were illegal, why are they not still under investigation?