The controversial American author Gore Vidal today brought his outspoken views on the Oklahoma bombing to the Edinburgh International Book Festival.

Vidal, who spent three years corresponding with the executed bomber Timothy McVeigh, suggested that FBI agents knew about the bomb, but did not warn people for fear of blowing their own cover.

Speaking at the festival to an audience of more than 400, Vidal said that a researcher had given him a "great deal" of information about the 1995 Oklahoma bombing that killed 168 people. He added: "I'm just about to drop another shoe in it. The researcher knows at least five people who were involved in the making of the bomb and detonation of it and it may well be that McVeigh didn't do it."

McVeigh's request to have his execution broadcast on national television was turned down, but he corresponded with journalists by email and invited Vidal, on assignment for Vanity Fair, to witness his execution. As Vidal said today: "What do you say to an invitation like that? No is really rude, and yes could be worse."

He had been fascinated by McVeigh's letters to him, and was astonished at how well-read he was. "Here we have a case where he was condemned to death and had two choices - one, he spends 50 years living in a box, or is executed," said Vidal.

"He seemed to have no fear of death and preferred execution, or as he said in one of his letters to me, state-assisted suicide."

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