Initial indictors suggest that the devices that went off on Thursday were put together in a way very similar to those used two weeks ago.
The rucksacks themselves, as well as the choice of three tube trains and one bus as targets all suggest a similar method of attack, say investigators.
They speculate that the devices were so similar to those used two weeks ago that they may even have been part of the same batch.
These are homemade explosives and their use is very difficult. They can be very volatile and sometimes the materials fail to go off.
There will need to be detailed chemical analysis of the substance that did not detonate this afternoon, in order to prove conclusively that it was indeed explosive.
However, police are working on the assumption that they were bombs intended to cause mass casualties.
The investigation will now centre on why the devices failed to detonate properly, what can be learned from them and what has become of the men seen fleeing from the scene of the different attempts.
The worrying thing in this is that we saw a huge and very energetic response from the police and from MI5 and all the
other relevant agencies when we had the large number of casualties a fortnight ago.
The state of alert issued by the Joint Terrorism Analysis Centre, which assesses the UK's terrorism threat, was critical - the very highest level of alert it can be.
All sorts of security precautions had been taken in the city to try to prevent further attacks.
Despite this, security services failed to prevent a group of people from planting bombs of a very similar design, possibly made in the same batch as those that were placed two weeks ago.
They have been able, apparently, to mount an almost identical attack - three bombs on the tube, one on the bus - and this is bound to be of great concern to people trying to catch the bombers.
It is only by the tremendous stroke of bad luck on the terrorists' part and good luck for the citizens of London, that these attacks did not succeed.
A lot is being made of the possible forensic utility of having these unexploded bombs.
During the IRA days, this was always something that investigators hoped for: an unexploded device yielding all types of forensic clues. Clearly, the harvesting of these clues has already happened.
The really critical element here is the fact that there are four individuals we know about who left the scene.