The hunt for London's bombers took a dramatic turn on Tuesday when police raided houses in West Yorkshire looking for links to four British-born men believed to have carried out the attacks.
Police raided properties in Leeds on Tuesday (Photo: Tim Stevenson)
The BBC's security correspondent Frank Gardner outlines the day's events.
What was the most significant development on Tuesday?
First and foremost the discovery of the explosives, the discovery that these were almost certainly four suicide bombers - the first suicide bombings in Western Europe.
Most worrying to authorities in this country is the discovery that this appears to be a cell of which they knew nothing, of so-called clean skins - people who simply were not on any kind of record or trace at all, people who were living so close to their neighbours but apparently carrying out this planned attack
Do the police think they planned this operation on their own, or had outside help?
It's almost inconceivable that these four bombers were acting alone. They must have had assistance from outside. A lot of them are thought to be quite young, one of them is thought to be as young as 19, too young to have attended the camps in Afghanistan.
Almost certainly they had a controlling hand, some kind of guidance from outside. If it was al-Qaeda their usual pattern is to send in an expert, possibly a bombmaker, possibly somebody to strategically advise them on targets, who would then withdraw and be out of the country before the bombs even go off.
How was the investigation able to move so quickly on Tuesday?
It's amazing given that a bomb blasts things to smithereens, what can be gathered.
Just the residue of powder, the direction in which the blast goes, fragments of metal, eyewitness accounts, CCTV, video footage on people's mobiles, anything can lead into this.
But also you have to look at what clues are being fed into it from other European countries which are also trying to get on top of these various networks. Then there is the missing person whose relatives phoned into the police.
So why the slow start?
We must remember that this investigation had to start from a much slower pace than the Spanish one. That had the great advantage of having an unexploded rucksack bomb, with a phone and a Sim card, plus a van and detonators. They had none of that in the London case.
Does anything surprise you about Tuesday's developments?
I do think it very strange that they were able to find these documents belonging to these bombers.
Maybe they thought they were going to paradise so it didn't really matter but it allowed the police to trace where they came from very quickly. It was a strange thing to do.
Where does the investigation go from here?
The most dramatic development is the discovery of a "significant amount of explosives" at an address in West Yorkshire. Then there's the arrest of one suspect, who will be questioned shortly at Paddington Green police station in London, almost certainly by members of the anti-terrorist branch.
But the investigation doesn't stop here. We heard from senior police officials that there will still be enquiries going on on the ground and searches of the scenes of crime, and there will of course be a continued massive international terrorism operation to try to join up the jigsaw that they are trying to put together.
Is the UK still at risk from attack?
Yes, I'm afraid the country is still at risk. Right now we are at threat level "critical", which is the highest level on the scale and what that means is that another attack could be imminent.
If these people died on the Tube, then that means they are not out there about to blow themselves up, which is what happened in Madrid just over a year ago - when the Spanish counter-terrorism people cornered the bombers and you could hear cries of "God is greatest" and then they blew themselves up and killed somebody with them. But they could have accomplices and supporters.