In a withering address at the Edinburgh book festival, the liberal novelist and elder statesman of the Gore political dynasty said the former soldier decorated for bravery in the Gulf war wanted to send out a warning that the government had been bought by corporate America and "its secret police, the FBI, were out of control. What McVeigh was saying was, 'The Feds are coming, the Feds are coming'. "

In his strongest identification yet with the man who confessed to blowing up the Federal Building in Oklahoma City in 1995, killing 168 people in retaliation for the FBI's "slaughter at Waco", Vidal described him as a "Kipling hero" with an "overdeveloped sense of justice" who did what he did because he was inflamed by the massacre, the FBI's subsequent cover-up, and the way it "had shredded the bill of rights and the constitution. He was the man who would be king."

Vidal, whom McVeigh asked to witness his execution in June after the pair corresponded for three years, insisted McVeigh did not actually carry out the bombing, and hinted he was now close to revealing the names of those who did.

"I am about to drop another shoe. I have been working with a researcher who knows at least five of the people involved in the making of the bomb and its detonation. It may well be that McVeigh did not do it. In fact, I am sure he didn't do it. But when he found out he was going to be the patsy, he did something psychologically very strange. He decided to grab all credit for it himself, because he had no fear of death."

Vidal maintained this was because "McVeigh saw himself as John Brown of Kansas", the anti-slavery campaigner who was executed after leading a raid into the south which sparked the American civil war.

Vidal alleged that the FBI not only knew about the plot, it was involved in it. Having infiltrated the rightwing militia group that planned it, it did nothing because it wanted to pressure President Clinton into pushing through draconian anti-terrorist legislation he was refusing to sign. "Within a week of the bombing, Clinton signed it for 'the protection of the state and of persons', using the exact language that Adolf Hitler used after the Reichstag fire of 1933."

America was in the grip of what he called "a revolutionary situation" because wealth had become concentrated in the hands of only 1% of the population. "The truth is that 80% are not doing well, and many of those are farmers out in the mid-west who have been driven off their land by big business. They are the backbone of the militia movement. Many of them are as crazed as you can find. But they number over 4m, 300,000 of which are active."

Vidal revealed that having had his last meal of mint ice-cream with chocolate sauce, McVeigh spent his last hours watching the Coen Brothers' film Fargo on a black and white TV. "It's a great film but bloody, a body is shredded and suchlike, and not quite what he wanted to see, poor fellow."

He saved his greatest venom for Janet Reno, the attorney general during the 52-day Waco siege, for "persecuting a perfectly harmless bunch of religious nuts" and for presiding over the "lies and cover-up" that followed it. "Her mother was a very famous alligator wrestler in Florida, a family profession she herself should have pursued."

© Guardian Newspapers Limited 2001

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