The spectre of the American nut turned guru, attended by a motley group of glassy-eyed disciples and holed up in a remote and well-armed compound, may seem at this stage just another quirky feature of the US landscape. But what happened on April 19th, 1993, in Waco, Texas, has attained the status of a permanent nightmare for many Americans; for Attorney General Janet Reno, the massacre at Waco has remained a private nightmare, say those close to her.

Again and again she has questioned her decisions and wondered whether she could have done anything differently, they say. Now, six years later, Waco is exploding again, and the revelations unfolding each day are undermining confidence in the federal justice department and the FBI, as well as raising serious questions about a government agency which may have chosen to ignore the law.

On Sunday morning, February 28th, 1993, agents for the US Bureau of Alcohol, Tobacco and Firearms, the AFT, a federal agency that oversees violations of those three items, attempted to execute search and arrest warrants on David Koresh and his followers at a residential compound in Waco, a remote area of Texas.

Koresh was the charismatic and deranged leader of a religious doomsday cult called the Branch Davidians. The ATF had learned that the compound had become a tinder-box of ammunition and explosives and firearms. There were also unconfirmed reports that some of the 25 or so children living with their parents at the compound were undergoing abuse of some kind. In other words, the ATF and the federal government felt it was time to act. You can be a religious nut in America, but the general idea is that you must do so without arming yourself to the teeth.

Within minutes of their approach to the compound, gunfire erupted. Four ATF agents were killed, six wounded, and an undisclosed number of Davidians were killed.

A 51-day standoff ensued between federal agents, led by the FBI, and the Davidians. It was both strange and frightening; some Davidians asked to, and were permitted to leave. Most chose to stay with Koresh, who began a dialogue with the FBI's hostage rescue team. One conversation between negotiators and Koresh lasted seven hours and 38 minutes.

Meanwhile, a phalanx of FBI agents in armoured vehicles were deployed along the compound's perimeter. Television carried it live.

There were two schools of thought on the government side, and they soon began to clash over strategy. One side, led by the negotiators, maintained that the Davidians had sufficient supplies to last out a long siege but that they would eventually surrender.

The negotiators pressed for no action against Koresh, citing the cult's inclinations towards fiery biblical destruction. Clinton Van Zandt of the FBI's National Center for the Analysis of Violent Crime, the so-called "Silence of the Lambs" unit, studied a note from Koresh which contained scriptural references to destruction by fire. Van Zandt wrote that a confrontation with Koresh could "bring this matter to a `magnificent' end in his mind, a conclusion that could take the lives of all of his followers and as many of the authorities as possible".

Others in the FBI, those who ultimately prevailed, feared a mass suicide and pressed for psychological pressure, harassment, and action. They severed electricity to the compound. They blasted music from loudspeakers 24 hours a day, hoping to wear down the Davidians.

On April 16th, Attorney General Reno refused an FBI request to blast tear-gas into the compound. But after a meeting with FBI officials the next day, Reno changed her mind after she was assured that the tear-gas was not pyrotechnic - in other words, that no incendiary devices that could cause a fire would be used.

On April 19th, the compound exploded in flames, bringing the standoff to the apocalyptic conclusion that Koresh had predicted. At least 80 people died, including at least 25 children. From the beginning, the FBI and Reno maintained that the Davidians ignited the fire, and were not in fact killed by the government.

Conspiracy theorists, militias, and those suspicious of the government have maintained otherwise ever since, broadcasting their views at conferences, in books and newsletters and on radio talk shows and through the Internet. The disaster at Waco became a seminal moment, and has provoked subsequent terrorism in the US. Timothy McVeigh, the convicted bomber of the federal building in Oklahoma, had made a pilgrimage to the Waco site, and carried a false driving licence with an issue date of April 19th.

Now, the efforts of two unlikely men have produced new evidence about the government's role in Waco. Micheal McNulty, a documentary filmmaker, and David Hardy, a former government lawyer now in private practice in Arizona, sued the government for access to documents and evidence related to the siege. After six years of fighting those demands, the government was finally forced to turn over documents stored in a locked warehouse in Austin, Texas.

Contrary to the FBI's assertions, it turns out that an incendiary device was fired into the compound on the morning of the blaze. The evidence comes from an infra-red audiotape made from an aircraft that was assigned to circle the compound that morning. On the tape, the commander of the FBI's hostage rescue team is heard authorising the use of the device.

Of even greater concern, McNulty asserted in his film, Waco: A New Revelation, which was nominated last year for an Academy Award, was the presence at the compound of members of the US army's Delta force elite military team.

The US constitution prohibits the use of military action against US citizens on US soil without the authorisation of the President.

Critics of the government, themselves dismissed as reactionaries, have long maintained that the military has participated in actions against Americans.

Now, the Pentagon has admitted that three Delta force members were at the compound, but insists they were there as observers. No one has addressed why the US military's presence would have been required at such an event.

The revelation is a disaster for the FBI and the US Justice Department. Mark Potok, an expert who monitors anti-government and paramilitary activity for the Southern Poverty Law Center in Alabama, told the USA Today newspaper: "This gives credence to the wildest and most controversial views of what happened at Waco. It makes it difficult even for mainstream America to believe what the government says."

A number of congressmen and senators are joining in the outcry. Senator Chuck Grassley of Iowa said the revelation points to a "cowboy culture at the top of the organisation".

For her part, a shaken Reno has promised that she is "dedicated to getting to the truth" of what happened at Waco. "I, along with many Americans, have been troubled by what has transpired."

Yesterday, Reno announced that she would appoint an independent investigator, similar to the process whereby Kenneth Starr was appointed to investigate President Clinton, to launch a full-scale inquiry into the events at Waco.

With so many guns in America, and so much violence, the idea that the US military and the federal government would turn its sights on even a group of deranged and misguided souls is itself explosive. If it emerges that this happened - and at the moment no evidence exists that those incendiary devices started the fatal fire - a sad chapter in American history will become even more disturbing.