The BBC's Rob Watson reports: "The FBI still insists the fire was the cult's fault"

The admission comes as Mrs Reno ordered a new investigation into the 51-day stand-off between the FBI and the Branch Davidian cult after it was revealed that agents fired potentially flammable tear gas canisters.

The leader of the cult, David Koresh, and about 80 followers, including 25 children, died.

For the past six years, FBI agents have repeatedly and categorically denied that they fired any incendiary devices capable of sparking the blaze that consumed the cult's compound.

Janet Reno: "I'll get to the bottom of this"

But on Wednesday, FBI spokesman John Collingwood said: "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."

The military-type canisters differ from civilian tear gas in that "the military canisters may have contained a substance that is designed to disperse the gas using a pyrotechnic mixture," Mr Collingwood said.

High-level contradictions

The revelations contradict congressional testimony of high-ranking Justice Department officials including that of Attorney General Janet Reno.

Janet Reno: "Very, very frustrated"

Saying she was "very, very frustrated" that the information had taken six years to emerge Mrs Reno pledged to "get to the bottom" of why it took so long for the FBI to admit to the use of the potentially incendiary munitions.

"It is absolutely critical that we do everything humanly possible to learn all the facts as accurately as possible and make them available to Congress and the public," she said.

But she added that she still believed the FBI was not responsible for the deaths.

FBI version of events

Officials, who requested anonymity, said two military tear gas canisters were fired hours before the fire began.

The 51-day siege ended with the deaths of more than 80 people

The canisters bounced off the roof of the concrete bunker and landed in an open field, according to these officials, and so were unlikely to have been the cause of the fire.

Independent investigators concluded the fire began simultaneously in three places.

Based on information from bugs planted on the compound, the FBI maintains that the Davidians started the fire.

Arson investigators found that gasoline, charcoal lighter fluid and camp stove fuel had been poured inside the compound.

Army involvement?

In other developments, the Texas Department of Public Safety told The Dallas Morning News members of the Army's secret Delta Force anti-terrorism squad were at the scene the day the compound burned.

The newspaper obtained a Defence Department document under the Freedom of Information Act that confirmed the presence of the Special Forces unit when the FBI used tanks to assault the compound.

The US military is barred from domestic law enforcement.