MI5 will come under intense public scrutiny in the New Year when the 7/7 inquest examines whether the spy agency could have prevented the atrocities.
The Security Service, as MI5 is officially known, faces a series of difficult questions about why it failed to investigate the terrorists from Leeds who carried out the 2005 London bombings when they came to its attention 17 months before the attacks.
Many relatives of those killed argue that intelligence officials should have followed up ringleader Mohammed Sidique Khan and his number two, Shehzad Tanweer, after they were seen meeting known terror suspects in 2004.
MI5 says it was swamped with potential leads at the time and did not have enough information to prioritise Khan and Tanweer over others.
The service also argues that pursuing these questions could undermine national security – and even help terrorists plotting other attacks.
Graham Foulkes, whose 22-year-old son David was killed by Khan in the Edgware Road bombing, predicted that MI5 would be exposed at the inquest as "pretty inefficient".
"This is very important on a number of levels – personally for me, because as a father I need to know what happened and how this was allowed to happen," he said.
"But also we as a country need to know what happened because we need to learn lessons.
"We have to have confidence in those who are charged with protecting us."
The attacks on three Tube trains and a bus on July 7 2005 by Khan, 30, Tanweer, 22, Hasib Hussain, 18, and Jermaine Lindsay, 19, were the worst single terrorist atrocity on UK soil.
As well as killing themselves and 52 others, the bombers injured over 700 people.
The inquest, sitting at the Royal Courts of Justice in London, has already heard evidence about the attacks at Aldgate, Edgware Road and King's Cross underground stations.
When it resumes on January 12, the inquest will look first at the bombing of the number 30 bus in Tavistock Square and later consider whether there were any failures of communication between MI5 and West Yorkshire Police.