A report examining what MI5 knew about the 7 July London bombers before their 2005 attacks is to be published later.
The parliamentary Intelligence and Security Committee report was ordered after a trial heard ringleader Mohammad Sidique Khan had been followed in 2004.
The fresh report is expected to detail the nature of MI5's surveillance of him and accomplice Shehzad Tanweer.
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An earlier report cleared the security service because there had been no intelligence the men were a risk.
The Intelligence and Security Committee (ISC), which reports to the prime minister, revealed in 2006 that Khan and Tanweer had appeared on the periphery of another investigation.
