Security officials in the UK are working hard to ensure everyone involved in the alleged plot to blow up airliners is accounted for, says BBC security correspondent Gordon Corera.
Security is stepped at Stansted Airport among others
There are these stories floating around the US that the authorities are looking for more people.
I have spoken to people here and they say they do not recognise the idea there are identified people they have been looking for that they have not found. They say that is not the case.
But while they believe they have found the people they identified in the run-up to this, they acknowledge it is possible there are people involved who had not been identified as being involved - and therefore could be still out there.
The authorities had been monitoring these people for some time - but they then had to move faster than they anticipated against them
They could be important figures.
How long we are going to remain at critical-alert state, with all this security, depends on how long it takes them to bottom out this investigation - establishing they know enough, have followed enough of the trails to make sure there are not significant players out there capable of carrying out a Plan B.
They are not saying there is a Plan B and there are other players - but they are saying they need to do more work before they are sure there are not.
It is only when they are at that point that the Joint Terrorism Analysis Centre will lower the threat level beneath critical.
[The police are] trying to find out the scope of the alleged plot, who other conspirators might have been, what the links might have been abroad, were there back-up plans, were there alternatives?
Pakistan is looking increasingly important. There are words coming out of Pakistan about a number of arrests.
Police felt they had to move immediately
Officials in Pakistan have told me there have been arrests in recent days and it does seem as if Pakistan might have been one of the factors that triggered the arrests.
The authorities had been monitoring these people for some time - but they then had to move faster than they anticipated against them.
One possibility is people were arrested in Pakistan related to this alleged plot and there were concerns people in the UK might have seen this and sped up - or gone underground - and therefore police felt they had to move immediately.
It all came down to the question how soon was this alleged plot going to happen?
The view was it was imminent - in a matter of days - therefore they had to act fast.