On April 19, 1993, before the embers of the Branch Davidian compound had cooled and many of the corpses could be recovered, FBI commanders delivered a version of the tragedy from which the nation's top law enforcement agencies would not waver for six years: The FBI never used devices that could have ignited the blaze.

"We did not introduce fire into this compound," Bob Ricks, the FBI's chief spokesman in Waco, Tex., told reporters. "It was not our intention that this compound be burned down."

In recent days, Ricks and other former and current FBI and Justice Department officials have said they were stunned to learn that the bureau fired a small number of potentially incendiary tear gas grenades at a storm shelter as tanks carried out a final assault early that morning, ending the siege after 51 days. Seventy-six people died inside the compound, a majority of them women and children.

Federal officials insist the tear gas rounds did not ignite the blaze, noting that the devices were fired hours before the conflagration started and that they were aimed at the nearby storm shelter, not the main building. Arson investigators concluded that flammable liquids had been poured around the inside of the compound.

But the disclosure has undermined the official narrative, prompting charges of a coverup and reopening inquiries inside the Justice Department and in Congress. Embarrassed officials have been forced to reexamine thousands of pages of congressional testimony, internal investigative reports, trial transcripts and legal pleadings--all because of one new fact.

Then on Wednesday, FBI officials produced video and audio tapes from that morning of the fire that they'd always said did not exist, even in court filings.

Now, investigators are asking two key questions: Who within the FBI knew pyrotechnic tear gas grenades were used on April 19? And why did they remain silent nine days later when Attorney General Janet Reno testified before the House Judiciary Committee that, based on the FBI and Army briefings she'd received, "it was my understanding that the tear gas produced no risk of fire."

At the time she authorized tanks to ram and tear-gas the Davidian compound, Reno had been on the job about five weeks. "I asked question after question," she told the committee, "about anything that I could think of, trying to elicit as much information as I could to make sure that we had fully explored everything."

When the Waco controversy resurfaced after the bombing of the Oklahoma federal building in 1995, Reno told The Washington Post: "After two years of review, nothing has given me any indication that the FBI misled me."

Last week, Reno could not conceal her anger at the belated disclosure, which she acknowledged had tarnished her credibility and that of the FBI and her department.

Where did it all begin?

A review by The Post of thousands of pages of public records related to the Waco siege shows that information about the use of the pyrotechnic tear gas grenades was either withheld from investigators or overlooked as early as April 21, 1993, when scores of FBI agents were initially interviewed by bureau inspectors for routine reports compiled after incidents.

One such document--an FBI Form 302--obtained by The Post, is the account of FBI supervisor Richard M. Rogers, who was in charge of the 50-member FBI hostage rescue team at Waco. Rogers issued orders at the scene in consultation with the FBI special agent in charge, Jeffrey Jamar.

Rogers's Form 302 mentions the two nonflammable forms of tear gas deployed that day. He specifically noted the use of 40mm "ferret" rounds--football-size plastic shells fired from M-79 grenade launchers--which, as he said, are "nonburning" and "produce no heat." But the six-page narrative makes no mention of the use of pyrotechnic rounds.

FBI special agent Richard Intellini, who commanded a Bradley Fighting Vehicle, told investigators that one of his men shot two tear gas grenades at the storm shelter at 6 a.m., but they bounced off, according to his 302 report. Intellini said that both were nonburning ferret rounds.

According to a transcript released yesterday by the FBI, Rogers authorized the firing of the pyrotechnic rounds. At the time, according to the FBI, agents were attempting to penetrate the storm shelter, which an arson report says was "partially roofed with plywood covered with tar paper."

Rogers's attorney, Bill Lawler, said Rogers had no comment. Reached by telephone, both Intellini and Jamar declined to comment.

Five days before the blaze, Reno was briefed by military officers and FBI officials including Rogers on the use of tear gas. Danny O. Coulson, a former deputy assistant director of the bureau who attended the key briefing, said in an interview he had never heard about the use of pyrotechnic rounds until two weeks ago, when he talked to Texas authorities and others investigating the Waco fire. His confirmation of the firing of the two pyrotechnic canisters to the Dallas Morning News prompted Reno and FBI Director Louis J. Freeh to call for the new investigation.

Asked how the military gas could have entered the FBI ordnance supply, Coulson--the founding commander of the FBI's Hostage Rescue Team--said in an interview: "I don't have a clue . . . Dick Rogers would know. . . . Someone in the chain of command would have to know."

Rogers testified for two days at 1995 House hearings on Waco, but no questions arose on the use of pyrotechnic gas grenades. He told the panel about tear gas delivered by the combat engineering vehicles, saying they were deployed for "two purposes: One is to make an opening to put a nonflammable, nonburning type of [tear gas] in that building; and second of all, we wanted to use them to make escape openings for those people."

The Justice Department's voluminous report on the FBI's actions at Waco notes: "The gas delivery systems the FBI used were completely nonincendiary." Delivered on Oct. 8, 1993, the report was written by a special assistant to Reno, Richard Scruggs, now a federal prosecutor in Florida.

Scruggs did not respond to requests for comment this week, but in an interview with The Post nearly three years ago he defended the thoroughness of his inquiry:

"We interviewed every single person on the [Hostage Rescue Team]--50. We interviewed every single person who had anything to do with Waco at all--over 900 people, even secretaries.

"We interviewed the firearms specialist," Scruggs said. "When FBI agents fire their weapons, they have to account for it. With the FBI weapons, all the ammunition was accounted for. It was accounting for gas canisters that was difficult."

Appended to the Justice Department report were the findings of a July 1993 arson investigation, conducted by four fire experts retained by the Texas Rangers. The arson findings make no mention of pyrotechnic grenades.

"We weren't told about them," said James Quintiere, a University of Maryland professor who analyzed how the fire spread and later testified at the criminal trial of the Branch Davidians.

Quintiere supported federal prosecutors' conclusions that the Davidians started the fire.

Arson investigators concluded that the "fire was not caused by nor was it intensified by any chemicals present in the tear-gasing operations."

Releasing the official inquiries in October 1993, Reno said: "These reports have been widely regarded as both candid and comprehensive; therefore I believe any additional inquiries are unnecessary."

But the questions didn't stop there.

In 1994, a Waco television reporter, Joe Calao, found footage of what appears to be smoke rising near the Davidians' storm shelter between 7:30 and 8:30 a.m. on April 19, hours before the fire started shortly after noon.

Calao said he consulted numerous tear gas experts, who concluded that the FBI must have been using pyrotechnic rounds that morning, based on the clouds of smoke and gas.

Calao said he contacted the Justice Department for comment, and then-spokesman Carl Stern told him the camera must have captured a cloud of dirt, not smoke. Stern said this week he was relaying what he'd been told by Scruggs and others who had conducted the official inquiry, that no burning tear gas rounds were used.

By 1996, Calao's footage came to the attention of attorneys who had filed a wrongful death suit for the relatives and estates of the dead Davidians against the federal government. A photograph of a 40mm pyrotechnic gas grenade found at the scene surfaced in the civil lawsuit, suggesting that the FBI had fired a type of tear gas ordnance it had never disclosed.

"Mere speculation," the Justice Department's lawyers said in March 1998, seeking to dismiss the civil suit.

Colorado filmmaker Michael McNulty gained access to more than 500 containers of evidence held by the Texas Rangers, and said he found various other potentially fire-causing devices. In November 1998, he presented his findings to Justice Department and FBI officials and asked for a response for his upcoming documentary, "Waco: a New Revelation."

He says no officials were interested.

"We are aware of no evidence to support the notion that any pyrotechnic devices were used by the federal government on April 19," Justice Department spokesman Myron Marlin told the Dallas Morning News on Aug. 23. "We've said that all along."

Staff researchers Nancy Shiner and Heming Nelson contributed to this report.

CAPTION: On April 19, 1993, the Branch Davidian compound in Waco, Tex., burns out of control after a long standoff with law enforcement teams.