JULIAN RUSH: If London had a lucky escape today, police investigators had a lucky break. Because these bombs did not explode -- and the reason for that may well tell police something significant about the organization or skill of the bombers -- a welter of clues has been left behind because nothing has been destroyed. Everything is up for grabs as one former forensic scientist put it to me today.
SIR IAN BLAIR: We do believe that this may represent -- may represent -- a significant breakthrough in the sense that there is, obviously, forensic material at these scenes which may be very helpful to us. So I feel very positive about some of these developments.
JULIAN RUSH: Combing the scenes for clues will take days. They can't start until they've been made safe, but already the lines of inquiry are clear. Was this a copycat attack or was it done by people linked to the bombs of two weeks ago, or by a separate cell?
One thing that makes a difference, though, is bombers are still alive, and forensic clues will almost certainly lead to them.
MAN: It started to smell like rubber or wire, and it got a bit worse, and then suddenly people were starting screaming and shouting and running.
JULIAN RUSH: Eyewitnesses will be a key part of the investigations -- not just those who may have seen the bombers, but what they heard or saw or smelled can be clues for forensic teams. So police were quickly on scene, appealing for witnesses.
Forensic science provided the breakthrough after the Madrid bombings. A partial fingerprint on a mobile phone SIM card led to at least some members of the cell who carried out the attack.
The key question in London today: Were these attacks linked to the ones of two weeks ago? Forensic clues will come from traditional sources, fingerprints and fibers on the rucksacks and hold-alls and their contents. But DNA can now be taken from the tiniest of samples. Just holding the handle of a bag leaves enough sweat to be useful.
And then there's the bombs themselves. What type of explosive, commercial or homemade? Police will be looking to see if it's the same as the explosive material found at the car at Luton Station a fortnight ago, or if it matches up with the bath of liquid and solid that's still causing concern in the house in Leeds that's been dubbed "the bomb factory."
And even though much was destroyed in the blast two weeks ago, fragments would have survived, chemical traces of explosive, wire fragments in timers. If any match to today's crime scenes, police will know they're dealing with the same group.
Today's attacks bear uncanny similarities, too. It was three tubes and a bus. The three tube lines hit today all go through King's Cross. They were nearly simultaneous. Rucksacks were used last time, and eyewitnesses speak of the same today. But there were crucial differences, too. Sorting out what matters and what doesn't is the task ahead.