PHILIP ELLIOTT, Associated Press

WASHINGTON (AP) — Shadowy outside groups ran an estimated $25 million in ads to shape state-level elections this year, and their full roster of donors is unlikely to ever be known.

That’s according to an analysis released Wednesday by the nonpartisan Center for Public Integrity.

While that advertising spending is a small slice of the $850 million spent in statewide races, the amount is still almost twice what outside groups shelled out during the last midterm elections in 2010.

Mysterious groups also ran twice as many ads as they did in 2010, the first election after the Supreme Court cleared the way for the ultra-rich, corporations and unions to quietly bankroll campaigns through anonymous groups.

The state that saw the most ads from opaque groups was Kansas, where Republican Gov. Sam Brownback prevailed in an uphill contest with Democrat Paul Davis. The state saw a total of 34,300 ads through Election Day on Nov. 4. Of those, 11,328 were from nonprofits whose finances can be shielded for years, if not forever.

One of the most prominent outside nonprofits in Kansas was the Alliance for a Free Society. The group incorporated in Delaware in July, meaning its tax disclosures will not be available until 2015 or later. And there’s no telling whether those documents will include the patrons or a list of other similarly vaguely named groups.