"Horowitz wrote that he will explore “allegations that Department or FBI policies or procedures were not followed” in connection with both [Comey] letters. When he is finished, his office will probably issue a lengthy report detailing what it has found, as it has done in other high-profile matters, though it is also possible he could recommend criminal charges for anyone found to have broken the law."
I don't think you know what a criminal referral means.
The IG referral is made to the AG. The AG decides to indict or not. An IG doesn't indict.
IG can only recommend action. The DAG handles personnel matters and the Criminal Division would make the criminal case.