Fifty-two people were killed by the bombers
A judge is to decide whether an inquest into the 7 July London suicide bombings should ask if MI5 could have prevented the terror attacks.
Victims' families want the inquest to ask why the intelligence agency did not investigate the bombers' ringleader Mohammed Sidique Khan further.
MI5 has said this evidence would give al-Qaeda an "invaluable weapon" and should not be disclosed.
Lady Justice Hallett said she would make a ruling in the next three weeks.
The judge, who will preside over the inquests, has also been asked to decide whether the deaths of the 52 innocent victims of the blasts should be considered at the same time as those of the four suicide bombers.
Relatives of those who were killed are opposed to this, saying it would cause too much distress.
The hearings are due to be held in October.
Bereaved families want to use the inquests to ask security service officials why they did not follow up Khan after he was witnessed meeting known terror suspects.
But lawyers for MI5 told the hearing at the Royal Courts of Justice that allowing this would aid those planning future terror attacks.
Neil Garnham QC, counsel for the Home Secretary and MI5, described the London bombings as "the deliberate action of evil and callous killers".
He added: "The appalling truth is, however, that there are people out there who applauded and celebrated this appalling act."
Patrick O'Connor QC, counsel for four of the bereaved families and 15 survivors, had earlier strongly criticised MI5's involvement in the 7/7 case.
He said the agency demonstrated flaws in its assessment policy, record-keeping and co-operation with other agencies.