Keppel isn't giving the specifics of his departure, but the rumors have been flying around town like fungoes at Salt River Flats.
Among the better ones:
--Keppel is cooperating with an unspecified law enforcement agency concerning an investigation into his former boss, Horne.
--Keppel just couldn't take working for the highly political Horne anymore, an elected official who had zero experience in the criminal-justice arena before his November 2010 electoral win.
For sure, Keppel didn't "retire"--he quit, and he doesn't have another gig lined up.
Whatever the real reason, the loss of Keppel is being universally viewed as an outright disaster to Horne's already shaky credibility as Attorney General.
Before he came on board with Horne in January 2011, Keppel had earned a reputation as impeccably honest and fair, both as a Maricopa County Superior Court judge (he once was the criminal presiding judge) and, before that, as a career prosecutor.
Among many, many other high-profile cases, he was lead prosecutor in the infamous early 1990s AzScam corruption case which ensnared several state legislators and other powerful figures. Keppel's work in securing numerous convictions in that sprawling case was the stuff of legend.
A good friend of Keppel's, who we won't name, says this about the 69-year-old:
"Jim is truly an ethical guy and he doesn't suffer fools gladly. I know he really respected and enjoyed a lot of the people he worked with at the AG's Office. And I also know that Tom Horne wasn't one of them, for reasons that eventually may emerge."