WASHINGTON — FOR one long week in April 1993, Attorney General Janet Reno consulted, mulled, and deliberated over what she now calls, ''the most difficult decision that I have ever made.''
The uncertainty was over how to resolve the six-week standoff at the Branch Davidian compound in Waco, Texas.
Ms. Reno finally endorsed an FBI plan to use tanks to pump tear gas into one section of the compound, in hopes of persuading the gasping Davidians to surrender.
Instead, a fire engulfed the wooden building, killing 81 of the people she intended to save, and charred the reputation of federal law enforcement.
When Reno testifies tomorrow at the House Waco probe, members of Congress will finally be able to ask the event's central decisionmaker some central questions: Did the FBI consider the possible toxic effects of the gas? Could the tanks or the gas have started the fire? And why did the FBI decide to move in on the compound after 51 days of negotiation?
In a briefing last week, Reno took full responsibility for the results of the assault, events she revisits in her mind ''again and again.'' But however badly Reno feels about the outcome at Waco, she is not about to hurl herself off Tosca's tower.
''As I go through all of this, I still reach the conclusion that I did then,'' Reno says, ''that faced with what I knew then, and faced with the best information I could get at the time, I did not see another option.''
Reno, who says neither President Clinton nor the FBI pressured her in any way, contends that of all the plans to end the standoff, use of the tear gas - known as CS gas - was the most viable.
''I could have gone in with a frontal assault, which would have been extraordinarily dangerous to all concerned,'' she says. ''I could have waited and I won't know and will never know what David Koresh might have done.''
According to Reno, FBI experts concluded that even though Koresh, the Branch Davidians' leader, had agreed to surrender after he finished his interpretation of the seven seals in the book of Revelation, there was no guarantee that the manuscript would be completed soon. Koresh, they noted, had broken previous promises.
FBI witnesses say they were concerned that Koresh might have come dashing out of the compound in an ''armed suicidal raid'' that would have taxed the resources of the special FBI tactical or ''HRT'' team at the scene.
''We were faced with a situation where the HRT team's readiness and ability to respond was being diminished after 51 days on the site,'' Reno says. Since then, she notes, she has added an additional HRT team and enhanced the SWAT (Special Weapons and Tactics) team so that if a similar crisis occurs, she would have the ability to withdraw one team and install another.
Jeffrey Jamar, a now-retired FBI agent who commanded the Waco operation, testified last week that the timing of the raid was justified, because it allowed the FBI to have all its personnel, plus emergency ambulances and helicopters, on the scene.
Harry Salem, a Pentagon toxicologist, testified that he told Reno CS gas is ''the safest and most potent riot-control agent we know'' and that it would not harm any of the small children in the compound. William Sessions, then-FBI director, said Reno was ''very, very interested'' in the decision and ''was particularly concerned about the children.''
University of Maryland arson expert James Quintere said Friday that in his view, neither CS gas nor the tanks contributed to the fire. Since the blaze began in three separate areas in the compound more than a minute after the last tank had withdrawn from the building, it must have been ignited by the Davidians themselves. Federal agents cited surveillance tapes before the blaze in which voices could be heard saying, ''Spread the fuel.''
But Republicans on the panel and some witnesses have taken issue with these contentions.
George Uhlig, a chemistry professor at the College of Eastern Utah, testified that CS gas can be toxic and flammable if used in large quantities in an enclosed space. The FBI decision to lob 400 football-sized canisters into the compound, he says, ''was not in the best interests of the children.''
Republicans on the panel criticized the FBI's decision to use tanks to batter ''escape holes'' in the compound's walls. Rep. John Mica (R) of Florida suggested that the tanks could have started the fire by crushing lanterns or propane tanks, or by causing such a panic inside that Davidians knocked them over themselves.
''Regardless of whether they died from the gas,'' Representative Mica says, ''they were suffocated by our actions, or incinerated.'' He called the gassing ''a flawed decision based on flawed advice.''
Davidian Clive Doyle testified Friday that the FBI erred in assuming Koresh and his followers never planned to come out and invented the notion that they had a suicide pact. ''We were trying to cooperate,'' he said. ''We had our bags packed.''
Why didn't more Davidians flee the compound during the final assault? Doyle explained that many feared they they would be shot if they fled, or else arrested and indicted for murder.
As Reno testifies tomorrow, Republicans will likely grill her about Mr. Clinton's role. Reno says she informed the president of her decision to use gas, and ''he backed me up.'' Nevertheless, Rep. Ed Bryant (R) of Tennessee argues that ''in the end, [Clinton] did have the ultimate authority as president ... to say no, let's not do that.''