Captain Walker Keel: Commanding Officer, U.S.S. Horatio. Suspected that Starfleet had been co-opted by a secret alien influence late in 2364. Brought Captains Rixx and Tryla Scott into his cabal, unaware that the latter had already been turned. Keel grew more paranoid and didn’t trust his first officer or his chief medical officer. Arranged for a secret meeting with Rixx and Scott and brought along Captain Picard. After confirming to his best of his abilities that Picard was still himself, he voiced his concerns to his old friend but without firm evidence, Picard wouldn’t go any further. Not long after the Horatio was destroyed, evidence suggested it was sabotage. There were no survivors. 

Appeared in TNG’s “Conspiracy”