In a two-part programme on Radio 4, BBC security correspondent Gordon Corera looks into the shadowy world of Britain's security services, forced into radical change after 9/11.
9/11 was the start of a dramatic turnaround for MI5
In the immediate aftermath of 9/11, only one plane was allowed to fly from Britain to the US.
On board were Britain's top spy chiefs, including Eliza Manningham-Buller, then number two at MI5, and soon to become the director general.
They dined for an hour-and-a-half with a handful of American counterparts at CIA Headquarters in Langley, Virginia, and offered their unqualified support.
Their long journey was appreciated by their American counterparts.
"They didn't have to do that. They could have called," remembers one of the CIA officials present.
The Americans revealed that they were already sure al-Qaeda was responsible, having recognised names off the passenger lists for the flights that had crashed.
They also said they believed the attacks were not yet over.
Back home, the atmosphere was less than calm amidst fears of a hit on the UK.
"The atmosphere was very frenetic back then in September and October - it felt like trench warfare," Ms Manningham-Buller later told parliamentarians.
As one observer put it: "After 9/11, they were chasing everything under the sun."
It was a dramatic turnaround for MI5.
After the end of the Cold War, there were questions about whether MI5 was really needed any more.
Bombs killed 52 people in London on 7 July 2005
In the 1990s, it wrested control of dealing with terrorism from Northern Ireland from the Police Special Branch and moved into fighting organised crime.
But it was only with 9/11 that its new role became clear - and even then it was not clear just how dramatic the transformation would be, a change which included being thrust in the public spotlight.
In the wake of the 7 July 2005 attacks, MI5 would face its most public barrage of criticism when it emerged that during the surveillance operation called Operation Crevice in 2004, its agents also had multiple sightings of two of the bombers who had gone on to kill.
The 7 July attacks also made clear that the threat was not coming from abroad, as had been thought in the wake of 9/11.
"The fact these there British citizens did really bring the system up with a jolt and had serious implications for what we were trying to do," says Sir Richard Mottram, who has just stepped down as chairman of the Joint Intelligence Committee.
A key task has been improving the flow of intelligence from communities.
The head of the Metropolitan Police Counter Terrorism Command, Peter Clarke, believes that this is now happening.
"I think perhaps in the period leading up to July 2005, there was a sense in some quarters that the threat had been exaggerated, it wasn't real, that it had been exaggerated for a number of reasons and that therefore there was nothing to give information about," he told me.
"Obviously that changed in July 2005 but I think as well that, the communities have been able to see that we are as a police service acting in a way which is entirely within the rule of law, we are acting independently and I hope they can see we are acting with integrity."
Last month, MI5's new director, Jonathan Evans, talked of 2,000 individuals who the security service believe pose a threat to national security, up by 25% from a year earlier.
MI5 is also growing fast - it will soon have doubled in size and will continue to grow. It has had to learn to work much more closely with the police as well as other intelligence agencies - both at home and abroad.
MI5: Under scrutiny - and under pressure
But as the infrastructure of counter-terrorism grows, have the mechanisms of accountability kept pace?
Who oversees MI5's work and makes sure they don't bug and burgle as they please, to use Peter Wright's phrase from Spycatcher?
"It is not a question of MI5 walking out on the street and deciding to bug and burgle people," argues former MI5 director general Stella Rimington.
Every time the security service wants to do something like break into someone's house to install a listening device or intercept their phone calls, it needs to get a warrant.
This has to be personally authorised by the home secretary.
The number of warrants has risen inexorably in the last few years.
Talking to former Home Secretary David Blunkett, you get the feeling he rather enjoyed asking questions, and even turning some of them down.
A commissioner also retrospectively checks a random sample to see the proper procedures were followed.
Ultimately, the home secretary is responsible for saying yes or no, but not for deciding what operations to run, a system designed to prevent security services being used as political tools, as has happened in some European countries.
But as the security service grows - and new technologies bring even more capabilities - making sure the mechanisms of public accountability and trust are in place is going to be vital.
The Real Spooks - MI5 and Counter Terrorism since 9/11 begins on BBC Radio 4, Tuesday 4 December, at 2000 GMT (repeated Sunday 1700 GMT).