The U.S. Interior Department's Inspector General's (IG) office sent their own investigators out here to investigate Chief Ranger Nelson in February. The IG oversees all the agencies in the Interior Department, including BLM. I've been told the Inspector General himself has their report, and what he will do with it is anybody's guess.
An investigation by a BLM Special Agent would most likely be a separate matter, conducted by BLM's office of Law Enforcement. This office, which issues and can revoke a Ranger's Law Enforcement Delegation, conducted an internal investigation of allegations against Chief Ranger Nelson last year. The results were inconclusive.
By the way, I have learned that yet another of Nelson's rangers is transferring out, which will leave Barstow BLM with only 6 rangers, compared with a full staff of at least 12.