Most likely a branch of the ACLU.
It's actually a stand-alone 501c3 that was started by a former Air Force JAG, named Mickey Wienstien (sp?). He's a Jew, who also happens to be an atheist. He cut his teeth suing the Air Force Academy (after he left the service), for alleging the proselytizing of recruits and cadets.
You might remember that the AF Academy was forced to construct an outdoor Wiccan "temple". I believe that was as a rusult of an action filed (in part) by this particular foundation.