A senior officer from the MI5 intelligence service gave evidence anonymously, and said that while two of the bombers had been on the agency's radar, they could not have been stopped.
Although officials initially had said they had no advance knowledge of the bombers, inquiries revealed that Khan and Tanweer had been under surveillance as part of an investigation into an earlier, foiled, bomb plot.
They were never pursued because officials were overwhelmed with other threats perceived to be more serious.
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