The inquests into the deaths of the 52 innocent people killed in the 7/7 bombings begins today.
More than five years after the attacks brought terror to London, bereaved families will finally have the chance to ask officials questions about whether their loved ones could have been saved.
The inquests have a wide-ranging remit to examine whether the emergency services' response was adequate and whether MI5 and the police could have prevented the 2005 atrocities.
But concerns have already been expressed about the Security Service's apparent reluctance to assist the coroner's investigation.
Graham Foulkes, whose 22-year-old son David was killed in the Edgware Road bombing, said MI5's attempts to keep details of its alleged failings secret were "really distressing" to the families.
The Security Service this week angered families by proposing that the coroner, Lady Justice Hallett, should be allowed to sit in closed session to hear highly classified evidence.
Mr Foulkes said: "By every kind of moral standard that you're brought up with, that's wrong. You're told, if you make a mistake, you hold up your hands.
"Here they are, drawing a salary to do a job which they clearly have not done. And they're employing every legal twist they possibly can not to be accountable. It really adds to the anguish that we're all feeling."