The episodes leading up to Langer getting shot, it appears he is new to NCIS, but prior episode it turns out he started out under Gibbs, then went to FBI. Now back? What gives?

He worked with Gibbs and Fornell on a case, he was still an FBI agent at that time, had been for 10 ou so years before he became an NCIS Enquêtes spéciales agent in season 6. Hope that helps