Official word from the WTS:
*** si p. 302 Study Number 4—The Bible and Its Canon ***
The Roman Catholic Church claims responsibility for the decision as to which books should be included in the Bible canon, and reference is made to the Council of Carthage (397 C.E.), where a catalog of books was formulated. The opposite is true, however, because the canon, including the list of books making up the Christian Greek Scriptures, was already settled by then, that is, not by the decree of any council, but by the direction of God’s holy spirit—the same spirit that inspired the writing of those books in the first place. The testimony of later noninspired catalogers is valuable only as an acknowledgment of the Bible canon, which God’s spirit had authorized.