Today, Londoners are remembering the attacks with somber memorials. And as they do, they are again asking what could have caused four young British Muslims to become suicide bombers -- and if there could be more attacks in the future.
"We left the [the King's Cross] platform, started going underground, approximately a few hundred feet [about 100 meters], there was an explosion, a flash of light, everything went dark, the train ground to a halt," one male eyewitness said in the aftermath of the attacks. "There was emergency lighting that came on, people started screaming, and then there appeared to be smoke or soot, it was everywhere and it was all over our clothes and our hands and we just had no idea what was going on."
More To Come?
The horror and confusion were magnified because initially no one knew just how many bombs had exploded around the capital and whether there were more to come.
British Prime Minister Tony Blair sought to reassure the public.
"It's reasonably clear that there have been a series of terrorist attacks in London," Blair said at the time. "There are obviously casualties, both people that have died and people seriously injured, and our thoughts and prayers of course are with the victims and their families."
The full scale of the attacks -- four separate bombings -- finally became clear only by the end of the day.
But the more complicated questions of who carried out the suicide bombings -- and why -- took weeks more to address.
Who Were The Bombers?
And some of those details have yet to be fully understood even a year later.
Investigators soon learned that three of the four bombers were first- or second-generation children of Pakistani immigrants. The fourth was a Jamaican-born convert to Islam residing in Britain.
The police also discovered that two of the bombers had previously traveled together to Pakistan, where they are believed to have received some form of operational training.
But it remains unknown whether they had contact with Al-Qaeda members in Pakistan or whether Al-Qaeda helped to plan the London bombings.
“It is not easy to find out what happened," Peter Clarke, head of antiterrorism for London’s Metropolitan Police, told the BBC in the run-up to the anniversary. "Such information as we do have does suggest there is probably a link to Al-Qaeda.”
And Why Did They Do It?
The reasons why the four young men – three of them British citizens -- became so radicalized against British society also are far from completely understood.
The men left behind few clues. But one of the suicide bombers, Mohammad Sidique Khan, proclaimed in a video made before the attacks: “We are at war, and I am a soldier.”
Al-Jazeera television this week showed a previously unseen pre-attack video of bomber Shehzad Tanweer saying Britons deserved to be hit.
He said his fellow citizens had voted for a government “that continues to oppress our mothers, children, brothers and sisters in Palestine, Afghanistan, Iraq, and Chechnya.”
Tanweer also warned that more attacks would follow his own suicide bombing.
The British government has not confirmed the authenticity of the videotapes.
A Looming Threat
As Britons today marked the anniversary of the bombing, they paid tribute to those who died in violence that no one anticipated on a sunny, July day one year ago.
And, amid the tributes to the victims, Londoners must ask themselves a frightening question. Could there be other such attacks in the future?
British police say they take the possibility very seriously.
"I think there will be further attacks, in fact I know there will be further attacks," Metropolitan Police Commissioner Sir Ian Blair said today.
He said that during the past year the police have prevented three further attacks.
"No matter how dreadful the terrorists are, it is infinitesimal compared to the strength of Londoners," he added.