Guardian report.
MI5 warned todaythat disclosing information on why it had failed to investigate the ringleader of the terror attacks of July 2005 could give al-Qaida plotters an "invaluable weapon".
Bereaved families want to ask security officials, during the coming inquests into the deaths of those killed in the London bombings of 7 July five years ago, why Mohammad Sidique Khan was not followed up after being seen meeting known terror suspects.
MI5 told a hearing at the royal courts of justice, set up to decide the scope of the inquests, that disclosing such information would aid those planning another atrocity.