IT IS A disturbing mark of the breakdown of Janet Reno's trust in the FBI that she dispatched federal marshals to collect new evidence from the bureau on the use of incendiary devices during the Waco siege six years ago. At one level, the move was simply designed to transmit the material, with the consent of the bureau, from the FBI to the agency most appropriate to safeguard it. But this could have been accomplished by less dramatic means. The move clearly includes an all-but-explicit message that the upper echelons of the Justice Department feel burned by the bureau and wanted to make that displeasure clear.

Ms. Reno's anger is understandable -- especially given that what precipitated her move was still more evidence that incendiary devices had, despite her orders and her later assurances to the contrary, been used. The mess the FBI created for her was hardly aided when FBI Director Louis Freeh suddenly discovered the virtues of having an independent investigation of the matter, as though it were somehow for the bureau to dictate the manner in which this investigation should be conducted. The current mess is the most dramatic of a long series of incidents that have betrayed a mutual distrust between the bureau and the Justice Department -- incidents that raise questions about their cumulative effect on law enforcement.

At this point, everyone seems to agree that an independent investigator is required for Waco. But of whom exactly should this investigation be independent, and how independent should it be?

Normally, the obvious choice to handle such a high-profile allegation of a major FBI foul-up would be the Justice Department's Office of the Inspector General. The IG has statutory independence, its own investigators and a track record of strong FBI oversight. But the office currently lacks a Senate-confirmed leader and is having serious budget problems. Moreover, it reports to the attorney general, and this investigation will have to settle questions related to the attorney general's own testimony. While it seems fairly clear that Ms. Reno herself was misled in this matter and not lying to Congress, any investigation would have to examine what she knew when she assured Congress that no pyrotechnic devices had been used. A vindication of the attorney general by an acting inspector general would not be credible to those predisposed to believe that she was more than a victim of bureau misinformation.

Any special investigator -- including an outside one -- also will ultimately report to Ms. Reno. But the appearance problem here can be muted. The key principle is that the investigation should not be staffed by FBI agents but by officers from other law enforcement agencies, and that there be a layer of insulation of the investigator from Ms. Reno for at least as long as her conduct is in any way at issue.

So far, there is no particular reason to doubt the FBI's contention that the tear gas canisters in question did not start the fire. That only makes the failure to produce this evidence years ago more mystifying. The bureau has a lot to answer for in this matter, and it is well worth -- at long last -- addressing the Waco raid in a manner that will preclude further second-guessing.