The agency is launching an internal inquiry into why it took six years to admit that agents may have fired potentially flammable tear gas canisters on the final day of the 1993 standoff with the Branch Davidian cult near Waco, Texas.

As the bureau reversed six years of categorical public denials that flammable devices were used, Attorney General Janet Reno and FBI Director Louis Freeh ordered 40 FBI agents led by an FBI inspector to re-interview everyone who was at the Waco scene.

They are to report ``within weeks'' on all aspects of the use of military-type tear gas and why it took so long to be admitted publicly, FBI spokesman John Collingwood said Wednesday night.

Meantime, Republicans in Congress made clear they would reopen hearings into the 51-day siege. The standoff ended with the death of cult leader David Koresh and many followers during a fire that erupted as FBI tanks pumped tear gas into their wooden main headquarters on April 19, 1993.

``We continue to believe that law enforcement did not start the fire,'' Collingwood said. ``Freeh is deeply concerned that prior congressional testimony and public statements (about the use of flammable devices) may prove to be inaccurate, a possibility we sincerely would regret.''

Although questions remain, Collingwood said, ``all available indications are that those rounds were not directed at the main, wooden compound. The rounds did not land near the wooden compound, and they were discharged several hours before the fire started.''

On Capitol Hill, Rep. Dan Burton, R-Ind., chairman of the House Government Reform Committee, said, ``I am deeply concerned by these inconsistencies. ... I intend for the committee to get to the bottom of this.''

Sen. Charles Grassley, R-Iowa, who chairs a subcommittee that oversees the FBI, said, ``This is a serious development in terms of further erosion of the FBI's credibility.''

Collingwood said answers recently prepared by the FBI's Hostage Rescue Team to questions from lawyers for Waco families and survivors suing the government ``suggest pyrotechnic devices may have been used in the early morning of April 19, 1993.''

``The FBI may have used a very limited number of military-type CS gas canisters on the morning of April 19 in an attempt to penetrate the roof of an underground bunker 30 to 40 yards away from the main Branch Davidian compound,'' he said.

Unlike the civilian tear gas used later, ``the military canisters may have contained a substance that is designed to disperse the gas using a pyrotechnic mixture,'' Collingwood said.

Officials said two military tear gas canisters were fired just after 6 a.m., six hours before the fire began. The canisters bounced off the roof of the concrete bunker and landed in an open field, according to these officials, who requested anonymity.

Nonpyrotechnic tear gas canisters had not penetrated the bunker, which was linked to the main building by tunnels, the officials said. The FBI wanted to clear out anyone hiding there.

For six years, FBI and Justice Department officials have insisted no incendiary devices were used when the FBI ended the siege by agents trying to arrest Koresh on firearms charges.

Earlier this week, former assistant deputy FBI director Danny Coulson acknowledged for the first time to The Dallas Morning News that the two canisters were fired.

Since then, several drafts of the statement read Wednesday by Collingwood were reviewed inside the FBI and by Justice Department officials, who privately expressed anger that the FBI had allowed Reno and other officials to issue categorical public denials for years. Some officials said the incident would harm the credibility of federal law enforcement.

Two officials suggested word of the military canisters might not have been relayed to top FBI and Justice officials earlier because original inquiries focused on the fire in the main building and the military canisters had been fired almost 180 degrees away from that building and hours before the fire.

Independent investigators concluded the fire began simultaneously in three places. FBI bugs recorded Davidians discussing spreading fuel and planning a fire hours before the compound burned. Arson investigators found that gasoline, charcoal lighter fluid and camp stove fuel had been poured inside the compound.