Citing excessively bureaucratic divisions between the CIA and the FBI, inappropriate surveillance methods, and an outdated approach to intelligence gathering as initial explanations for failing to intercept the worst single terrorist attack on the nation, congressional leaders and experts called for an overhaul of the way in which the intelligence agencies operate.

"It's troubling to all of us in America, I suppose, that nobody had a clue as to this forthcoming attack of such devastation," said Senator Richard Shelby, the Republican vice-chairman of the senate intelligence committee. "It was a lot of people involved, a lot of coordination involved. There had to be some evidence, somewhere, of something being planned. When you have no inkling, no idea, not one bit of evidence that something of this magnitude was going on in the United States of America, then you've got an intelligence failure."

Mr Shelby is one of a growing number seeking the resignation of the CIA chief, George Tenet, arguing that the task should be elevated to cabinet rank and given to a more capable person.

"I personally like George Tenet, but there have been too many failures on his watch and this is a big failure," Mr Shelby said.

In an opinion poll, 59% of Americans blamed the CIA for the attacks and 52% blamed the FBI.