Does the trial court have any authority to prevent the defense from pursuing a line of inquiry into whether a third party was involved in the murder of Michael Treto?

The trial court did not abuse its discretion by precluding that line of inquiry. The defense did not offer any evidence indicating a third party had committed the murder. (See People v. Bradford, supra, 15 Cal.4th 1229, 1325, 65 Cal.Rptr.2d 145, 939 P.2d 259.) Moreover, although the defense was informed it could subsequently reintroduce the subject of the letters upon further development of a theory of the murder related to Treto's purported involvement with such a third party, counsel did not do so.