Google the words "Kochs" and "ACLU" and you will get more than 5 million hits. Most of the top ones reference a $20 million dollar donation that conservative philanthropists David and Charles Koch gave to the American Civil Liberties Union a decade ago to challenge the PATRIOT Act.

The gift is often cited by conservative and libertarian blogs to defend the Kochs from their liberal critics, since it shows that the brothers are more than just funders of conservative causes.

There's just one problem: The donation may never have happened. The whole thing may be an urban myth.

I first started looking into the matter last week while reporting on a $25 million donation Koch Industries and the Charles Koch Foundation made to the United Negro College Fund, an announcement that seriously upset some liberals. I recalled the Kochs' earlier ACLU donation and thought it should also be referenced in my story since it seemed to be of a piece with the UNCF donation.

This proved to be a whole lot more difficult than I thought it would be, since I quickly learned there was no primary reference for the donation. That is, there was no announcement or other mention of it on either the ACLU's website or the Koch Industries' media site. I contacted both organizations and neither would discuss the matter.

"We only publicly acknowledge supporters who have given us permission to do so," ACLU spokesman Stephen Smith told me via email. "[W]e can neither confirm nor deny the Koch brothers' support."

He further explained: "If we were to also be able to state that someone isn't a supporter, then we'd be in an impossible situation to comment on someone who is a donor but hasn't given us permission to acknowledge their support. "

Smith suggested I direct my questions to the Kochs themselves, but a spokesman for them declined to comment on the matter. That seemed strange since, as the UNCF donation showed, the brothers are not always shy about acknowledging their largess.

Ultimately, I referenced the alleged donation in my article but was careful to point out that it's existence could not actually be confirmed. Others who have looked into the matter were never able to confirm it either.

Daniel Schulman, author of the extensive new biography Sons of Wichita: How the Koch Brothers Became America’s Most Powerful and Private Dynasty, doesn't think the donation ever happened.

"I brought it up off-handedly at one point with a high-level Koch executive, who gave me the strong impression that there was no ACLU contribution. His words to me were to the effect of: 'I'm not sure that that happened.' The 990s of the Koch family's foundations don't appear to reflect this gift either," Schulman told me via email.

Most online citations refer back to a February 2011 blog posting by the libertarian site

The posting, by then-Senior Editor Radley Balko, is titled "The Koch Brothers' Right-Wing Conspiracy to Undermine the PATRIOT Act." It was part of an online debate between Balko and liberal writer Jonathan Chait and meant to refute the claim that the Kochs only support Republicans and right-wing causes.

"[T]hat $20 million appears to be substantially more than the Kochs have contributed to all political candidates combined for at least the last 15 years," Balko wrote. He did admit to some uncertainty on his sourcing though, noting that his information came "secondhand" and adding: "I'm waiting to hear back from the ACLU for confirmation."

Three days later Balko wrote a follow-up post, conceding that his efforts to confirm the donation had been fruitless. That post doesn't turn up as readily in Google searches.

The original Reason post's two sources were two fairly obscure websites. One was New York Social Diary, which is devoted to reporting gala high-society events. The other was Faces of Philanthropy, a site which mainly profiles large celebrity donors, like Angelina Jolie and Leonardo DiCaprio. Neither site is, to put it bluntly, an acknowledged, go-to journalistic source for raw data.

A December 2008 post by New York Social Diary states that "(David Koch) and his brother Charles, along with George Soros, the Ford Foundation and the Rockefeller Foundation each contributed $10 million to the ACLU to defeat parts of the USA Patriot Act." There is no link for the information or any source cited. It simply asserts the donation was made.

Editor David Columbia told me in a Thursday email: "I don't recall my source on that donation. I’m sure it didn't come from David (Koch) himself but it must have come from someone who either is close him or George Soros. That could be several people I can think of. I am sure if I printed it that I was certain of my source at the time. Hope this is helpful." Columbia told Balko almost exactly the same thing in the follow-up Reason post.

Faces of Philanthropy's website states in a story posted on, ahem, April 1, 2010, that "David Koch has also contributed $10 million to the ACLU in relation to crushing Parts 15, 16 and 17 of the US Patriot (sic) Act." The reference includes a link the Koch Industries Wikipedia page and no other sourcing. It is possible that it's information originally came the New York Social Diary report, which came out about 15 months prior.

I have not been able to reach anyone at Faces of Philanthropy. editor Nick Gillespie told me Friday: "I think that Radley's follow-up, where he says he was not able to get confirmation of the donation, stands as a pretty good correction or at least puts a question mark around the piece."

David Koch, as it happens, is on the board of trustees for the nonprofit Reason Foundation, which publishes the magazine. I am familiar with the magazine's editors, having been an occasional contributor, and have no reason to doubt their journalistic integrity.

To sum up: I cannot conclusively prove the donation did not happen, mainly because the people directly involved will not talk. But there are apparently only two sources for the claim and there is plenty of reason to doubt both. The only source who will speak cannot remember where he got the story. Others who have looked into it have not been able to confirm it. Taken together, there's just no compelling evidence to believe that the $20 million donation ever happened.