A court in London began hearing inquests into the deaths of 52 people killed in a series of bombings that targeted London's transport network on July 7, 2005.
The inquiry will examine whether the police or the MI5 domestic intelligence service could have intercepted the bombers, two of whom had been under the surveillance of the authorities.
The hearing at the Royal Courts of Justice opened on Monday with a one-minute silence to commemorate the victims of the three simultaneous blasts on metro trains in the UK capital and a later explosion on a bus.
Families of victims and survivors have failed in their calls for a full public inquiry into the attacks. But the inquests will provide the first chance to challenge official accounts, which the families have labelled as insufficient, inaccurate and misleading.
"It is disgraceful that there has never been a public, judicial examination of all the facts which is truly independent of the government, the police and the security service," Clifford Tibber, a lawyer whose firm represents families of six of the victims, said.
"These inquests represent the first opportunity for a public examination of the facts and to consider, if there were failings, what lessons have been learned."
Inquests are fact-finding inquiries that take place when a person dies violently or under unusual circumstances.
In the immediate aftermath of the bombings, officials stated that the four men - Mohammad Sidique Khan, Shehzad Tanweer, Hasib Hussain, and Germaine Lindsay - were unknown to authorities.
But in the years since the attacks, it was revealed that two of the bombers had been on the radar of the security services, but were not deemed significant threats.
||Shehzad Tanweer kills seven and injures 171 on train between Liverpool Street and Aldgate East
||Mohammed Siddique Khan kills six and injures 163 on train at Edgeware Road
||Germaine Lindsay kills 26 and injures 340 on train between King's Cross and Russell Square
||Hasib Hussain kills 13 and injures 110 on a bus in Tavistock Square
Evidence at a number of court cases has shown that Tanweer and Khan, the organisers of the attacks, were photographed, recorded and followed by intelligence operatives several times in early 2004 in the company of plotters later jailed for planning other attacks using fertiliser-based bombs.
"I want the inquests to look at whether any mistakes were made or flawed systems were in place," said Ros Morley, whose husband Colin, died in one of the metro bombings.
"Innocent citizens in the UK and worldwide need to know that they are protected now and in the future."
A report by the parliamentary Intelligence and Security Committee last year said that despite being identified by MI5, the two men were not considered "essential" targets.
It concluded that the the intelligence service could not have stopped the bombings because it lacked the resources to investigate Khan properly.
Al Jazeera's Tim Friend, reporting from outside the court, said that the inquests have been delayed for five years because of a number of criminal cases that had been taking place in the UK.
"There have also been arguments about precisely what areas this inquest should cover because the security services are worried that intelligence matters do not become public," he said.
"But the coroner is keen that at some point this inquest does cover that territory."
MI5 has sought to block much of the questioning on the grounds that it would require the disclosure of secret files that would threaten national security.
The legal battle over whether the confidential information should be heard at the inquests is not expected to begin in full until next year, with the early part of the hearings set to be largely procedural.
In a video found after the bombings, British-born Khan criticised Western governments for attacks on Muslims.
"Your democratically elected governments continually perpetuate atrocities against my people all over the world. And your support for them makes you directly responsible," he said.
Source: Al Jazeera and agencies