By Gordon Corera
BBC security correspondent
Despite last Friday's intense activity and important breakthroughs, the police are making clear that their investigations into the 7 July and 21 July attacks are far from over and there remains a need for urgency.
Police are trying to assess what kind of threat still remains
Even with suspects in custody and being questioned, there's still the need to collect evidence for possible prosecutions, including forensics from recent raids.
But there's also the continued crucial need to understand what might have been behind the attacks.
Who supported the suspected bombers? Who financed them? Who supplied them with explosives or instructions on how to make them? And who recruited them?
These questions are clearly critical in understanding what kind of threat remains out there.
Could any network have supported other cells or groups?
'No specific intelligence'
There's also the question of why explosives were left in a car at Luton railway station.
Police say their heavy presence at Tube stations is about reassurance
Officials say they have no specific intelligence about other threats. However, that doesn't preclude more general intelligence about a further threat.
The heavy police presence at Tube stations is officially about reassurance but could also be about deterrence. Even without any specific intelligence, given that the 7 July and 21 July attacks came out of the blue, there's no reason why another cell could not also appear out of the blue and launch a further attack.
The UK's threat assessment was raised to its highest level - critical - after 7 July and it has stayed there.
It remains unclear whether there are any links between 7 July and 21 July and how deep they might be.
Similar types of explosives appear to have been used but not necessarily from the same batch, and they may well have been manufactured separately.
One possibility is that a planner tasked both groups to carry out attacks and gave them instructions but that there was no direct contact between any individuals in the groups.
One figure who has been named in some reports as a potential mastermind and who has now been arrested in Zambia is not thought to have played an active role in the way that some accounts have claimed.
There continues to be a belief that an international link may well be found but the overseas aspect of the investigation has proved to be slow going.
Reconstructing phone records has been vital
The investigation into the 7 July bombers travel to Pakistan has been hampered by political and diplomatic sensitivities and a lack of clear evidence.
There has been speculation about links into Saudi Arabia because of reported communication between suspects and individuals in the Kingdom. But investigators are stressing that they are looking at hundreds of contacts and communications and it would be wrong to single out any particular one as having significance at this time.
If, however, the contacts are with known al-Qaeda figures it could dramatically change the focus of the inquiry.
Reconstructing phone records has been vital and leads have been passed on to countries like Pakistan for them to try and track who suspects may have contacted.
But there is a considerable difference between establishing some kind of contact and then understanding what the significance of that contact actually was.
Investigators draw concentric circles of contacts emanating from key suspects but as these get wider and wider, they include more and more individuals. In these circumstances, it's hard to know the nature of any relationship and whether their presence in the wider web is just chance or the sign of a broader network.
Little is also understood about the way in which individuals were radicalised and recruited in the UK - where it happened and how.
Again, this is vital for understanding any broader network and identifying other individuals who might be involved in further attacks.
Huge progress has been made by the police and security services over recent weeks but there will still be concern about what they don't know rather than what they do know.
And until more of the questions are answered, the concerns over further attacks will not go away.