On Monday 30 April 2007, five men were convicted of terrorist offences relating to a plot to detonate a fertiliser bomb in the UK in 2004. The arrests were the result of a police and MI5 operation codenamed CREVICE. Following the trial, the media reported that, at the time MI5 had been investigating CREVICE, the bomb plotters had been in contact with two unidentified men now known to be Mohammed Siddique Kahn and Shazad Tanweer, two of the four men who, on 7 July 2005, detonated bombs on the London transport system, killing 52 people and injuring several hundred others. This report investigates why MI5, knowing of Khan and Tanweer, did not prevent the 2005 bombings. Part A examines what happened in Operation CREVICE and subsequently. Part B describes when MI5 came across some of the 7/7 bombers and the questions these events raise. Part C of the report considers the wider picture and lessons to be learnt. The CREVICE conspirators made over 4,000 telephone-based contacts and met many people. Throughout 2004 and 2005 these were being investigated by MI5 as they pursued other plots and unearthed still more people of interest on the sidelines of each plot. Although Khan and Tanweer were amongst those of interest, though still unidentified, they were never put under surveillance as, based on what was known about them at the time, they did not merit resources being diverted to them (as opposed to other individuals known to be involved in attack planning). The Committee cannot criticise the judgments made by MI5 and the police based on the information they had and their priorities at the time. An update to the report outlines the reason for the delay in publication pending completion of other legal proceedings and gives further evidence uncovered recently.