SINCE THE 1993 conflagration at the Branch Davidian compound in Waco, Tex., the FBI has steadfastly maintained not only that law enforcement agents did not start the fire but also that they did not send any incendiary devices into the compound that could have done so. Attorney General Janet Reno has said as much before Congress. Suddenly, more than six years after the raid, the FBI has admitted that incendiary tear-gas shells were used after all.

The government now concedes that two such canisters were fired hours before the blaze erupted but contends they were not fired at the main building and had nothing to do with the deadly fire. This may be true, but the admission is nonetheless a stupefying development that calls into question the integrity of the previous investigations of the Waco siege and will arm the conspiracy theorists who believe the Branch Davidians were innocent victims of oppressive government. How exactly is it that in all the earlier studies, these tear-gas shots have never been accounted for or admitted?

Somebody must have known as the attorney general was assuring Congress that no incendiary devices were used that she was misinforming members. Ms. Reno put it succinctly yesterday in announcing an investigation: "I don't think it's very good for my credibility." The attorney general stated that prior to the assault, "I received assurances that the gas and its means of use were not pyrotechnic. Since then, I have consistently been told that no pyrotechnic devices were used." On a matter this central to such a high-profile and longstanding case, how could she and the FBI hierarchy have been misinformed for so long? The public is entitled to a full explanation -- and quickly.