By Gordon Corera
BBC security correspondent
Progress in the inquiry into the 21 July bombings has been highly visible with arrests in London and Rome and the first related charges. But the investigation into the deadly 7 July attacks has been far less visible recently.
The attacks are still being treated as separate inquiries
Many of those arrested in the last two weeks are understood to be associates of the main 21 July suspects who are being investigated for any possible knowledge or involvement.
However, no-one is in custody in the UK for playing a similar role in the bombings that occurred two weeks earlier.
The known bombers of 7 July and failed bombers of 21 July also appear very different in terms of professionalism - and not just because the latter's devices did not kill anyone.
There's also a marked difference in terrorist 'tradecraft' - the ability of individuals to operate outside the knowledge of the authorities.
Investigators believe the 7 July bombers do appear to have been more self-contained and more careful. In turn that's making it harder to understand the plot.
Whilst there are obvious similarities between the two sets of attacks, it is still being seen as too early to come to any final conclusions about definite links.
If anything there has been a shift to expecting looser rather than stronger links if they are eventually found.
The two incidents are still being treated as separate inquiries by the police although there is close sharing of information.
There has been a considerable focus on building up a picture of the four men who killed themselves on 7 July - who they were, what they did, who their associates were and why they did what they did.
Progress has been made here as investigators try to complete a picture which would explain their radicalisation and recruitment.
This is considered vital to unpicking the broader network behind the bombers - and understanding if it supported other cells in the UK.
The international side of the investigation continues to be extremely active, with Pakistan a primary focus.
Co-operation has also improved with the Pakistani authorities, after some early political and diplomatic prickliness.
Investigators are still trying to track who two of the 7 July bombers might have met during their joint visit.
Possible travel to Pakistan at the same time, of one of those suspected of involvement on 21 July, is also being investigated as one of the leads that could possibly link the two incidents.
Police have been tracking down contacts of the 7 July bombers
Tracking down the international contacts of the 7 July bombers is a complex task.
Much of it is done through mobile phone records which have to be carefully analysed and followed up.
Unsurprisingly, the individuals may have called, contacted or met hundreds of individuals in the preceding months - many of whom may have been fellow radicals.
However, attributing significance to any of these contacts is a different matter and many leads are examined carefully but then ruled out as not having any operational bearing on the inquiry.
In the meantime, the UK's Threat Assessment remains at its highest level amid fears of another cell on the loose.
Security sources say there is no specific intelligence of a third cell but that does not preclude one's existence.
After all, the first two groups attacked out of the blue so there's no reason why another one could not do the same.