The failure to discover any signs of a "chemical device" following a police raid in Forest Gate is forcing both the police and MI5 on to the defensive.
By Gordon Corera
Security correspondent, BBC News
Police and MI5 deny a rift over the Forest Gate raid
It also suggests the tough decisions that have to be made on sometimes questionable intelligence - and the potentially damaging consequences when things go wrong.
The raid was based on a tip-off from an informant that some kind of device was being prepared.
It's the job of the security service MI5 to focus on gathering intelligence but it's the task of the police to act on this information by conducting raids and making arrests.
On the day of the raid, the head of the anti-terrorist branch of the Metropolitan Police, Peter Clarke, made a statement which hinted that the police were far from totally convinced that they would find something.
"The intelligence was such that it demanded an intensive investigation and response," he said.
"The purpose of the investigation, after ensuring public safety, is to prove or disprove the intelligence that we have received.
"This is always difficult and sometimes the only way to do so is to mount an operation such as that which we carried out this morning."
In an ideal world, the response to a piece of intelligence which is not clear may be to intensify covert surveillance to verify and learn more about whether it is valid or not.
An example of that occurring came from Canada the week after the Forest Gate raid.
Met Chief Sir Ian Blair has faced criticism over the raid
A group of individuals was watched for months, possibly even for more than a year.
They were tracked through their communications on the internet and the phone as well as through their movements.
It was only when they were alleged to have been in the process of buying the material for an actual bomb that they were rounded up by police.
However, the problem comes when the intelligence points to an imminent terrorist plot in the making.
In the Forest Gate case there may not have been the option of spending days and weeks checking out the validity of the story.
It has been hotly debated whether Mohammad Siddique Khan should have been investigated more intensively
This, according to police and security sources, was the dilemma faced in Forest Gate and the reason why, they say, they would act in the same way in the future faced with the same information.
Intelligence, as memories of the Iraq-WMD issue reminded everyone, is often fragmentary, elusive and sometimes plain wrong. It hardly ever allows 100% confidence.
In the case of counter-terrorist operations, it often comes from human informants or from monitoring of communications - when the subjects of surveillance may use ambiguous code words to try to hide their true intent.
Informants can be vital but they can also be unreliable if tainted by their own agendas, as some suggest could have occurred in Forest Gate.
'Shoulder to shoulder'
There has been speculation that something of a blame game may now be emerging between the police and MI5 over the intelligence that led to the raid.
The house in Forest Gate was raided by more than 250 police
Officially, that's being denied with sources saying the two organisations stood "shoulder to shoulder" and the decision to act was taken jointly with no wrangling.
But there is no doubt that the failure to find anything is not just embarrassing but also likely to have a serious impact on community relations, particularly since a man was shot during the raid.
And some people have been critical not so much of the decision to act in Forest Gate as the way in which they acted - especially the use of 250 officers to raid one house.
But the counter-question asked by officials is: What would have been the consequences of not acting on that kind of intelligence?
One important legacy of the 7 July bombings was the awareness among officials that if there is another attack they will come under intense scrutiny over whether they had missed any warning signs.
It has been hotly debated whether one of the 7 July bombers, Mohammad Siddique Khan, should have been investigated more intensively before the attack, following the unearthing of limited intelligence regarding his activities.
The official Intelligence and Security Committee investigation found that the decision not to investigate him further was understandable but that doing so might have helped prevent the bombings.
The result of this experience is bound to engender a concern among counter-terrorist officials that they need to make sure not to miss any future warning signs.
If intelligence leads to repeated raids where nothing is found, communities could quickly become alienated. But it is their continued co-operation that remains vital.