What I found was that these threats of military veterans joining "Right-Wing" extremist groups are overblown.
But Reuters published a story Aug. 23 titled "US Army Battling racists within its own ranks." Inside this report, the only real, hard statistic I could find, and the one fact which the whole story stood upon, was this:
"No one knows how many white supremacists have served since then. A 2008 report commissioned by the Justice Department found half of all right-wing extremists in the United States had military experience."
For the rest of the story, we're treated with evidence that's at best anecdotal, and at worst seems to point at what's called a "lone wolf" problem; meaning extremists, without any official militant group affiliation or enrollment, take things into their own hands — much as we saw with Wade Michael Page.