Could the 7/7 bombings in London have been prevented?
One of the critical questions that the 7 July inquests will consider next week is whether MI5 could have identified two of the bombers before they carried out their murderous attacks on the Underground and a London bus in 2005.
Had they been able to identify the leader, Mohammad Sidique Khan, and his accomplice, Shehzad Tanweer, and then established what they were planning, it's possible the attacks could have been prevented.
Fifty-two people were killed, and more than 700 injured.
A key moment in MI5's surveillance operation happened on 2 February 2004, 17 months before the London attacks.
Undercover officers were monitoring a terrorist cell of young British Muslims who, it later transpired, were plotting to bomb the Ministry of Sound nightclub in London and Bluewater shopping centre outside the capital.
The surveillance operation was codenamed "Operation Crevice".
The officers were watching the leader of the cell, Omar Khyam, and saw him talking to two other "unidentified males". It later transpired that the two "UDMs" were Khan and Tanweer, although their identities were not known at that precise moment.
At the time Khyam was suspected of being involved in sending men and money to Pakistan to support the jihad against American and allied forces in Afghanistan.
The two "UDMs" then left in a Honda Civic and drove north. MI5 officers followed them 150 miles up the M1 to West Yorkshire, stopping off to photograph them covertly as they filled up with petrol at Toddington Motorway Service station.
They noted the addresses in Leeds and nearby Dewsbury where the occupants of the Honda were dropped off.
Crucially, MI5 did not immediately notify West Yorkshire Police Special Branch that they had followed suspects associated with the Crevice cell onto their patch.
But although Special Branch knew nothing about this, the FBI in Washington did. We can reveal that MI5 was sharing intelligence and information with the FBI as it happened.
The FBI's senior agent in charge of the American end of Operation Crevice, Art Cummings, was communicating three times a day with his MI5 opposite number at the British Embassy in Washington, so he knew everything that was going on in real time.
The FBI was concerned that elements of the Crevice cell might also be planning to attack the US.
In a remarkable exclusive interview, Art Cummings told me he was concerned that there might be another cell in the UK.
"The fact that the core [Crevice] group were talking to some people travelling outside of the area - and I believe it was to the north - and that needed to be defined, because if the operation goes down early, then you leave this bad spot that can come back and haunt you later," he said.
That is precisely what happened. Art Cummings's concerns turned out to be justified.
Plotting to attack
On 6 February 2004, four days after MI5 had followed the Honda up the M1, alarm bells rang at MI5 Headquarters in London.
They learned via an electronic intercept that Khyam had been asking a member of the cell from Luton about the formula to make an explosive device based on ammonium nitrate fertiliser.
The two had been together the previous summer at a terrorist training camp in Pakistan. They had been instructed in how to make a bomb, but Khyam had forgotten the exact composition of the chemical mixture.
MI5 knew at once that they were no longer dealing with a cell that was facilitating jihad in Afghanistan but with terrorists plotting to attack the UK.
Yet they still did not tell West Yorkshire Special Branch that they'd followed their possible associates up the M1 only four days earlier, and that the cell with which they might be connected were potentially terrorists with serious intent.
West Yorkshire Special Branch were subsequently given the details of the car's number plate and the addresses where the occupants of the Honda had been dropped off.
They were tasked to carry out checks but were never asked to mobilise their experienced surveillance teams to keep a detailed watch on the suspects.
It was more than four months after Khan and Tanweer had been followed up the M1 that MI5 put West Yorkshire fully in the picture - and that was two months after the Crevice cell had been arrested.
'Need to know' basis
It's reasonable to suspect that the arrests would have alerted Khan and Tanweer and made them more circumspect about their activities.
So why wasn't West Yorkshire Special Branch brought on board much earlier, and asked to play a potentially critical role in putting Khan and Tanweer under surveillance?
The simple answer is that it didn't work like that at the time.
MI5 would only give Special Branch specific tasks to carry out within certain parameters and on a "need to know" basis.
But all that has now changed. The relationship between MI5 and regional Special Branches is different, following the establishment of Counter Terrorist Units around the country in which MI5 and the police work side by side and share intelligence.
Although the CTUs had been planned before the London bombings, the attacks accelerated their implementation. The question is whether things would have been different if the units had been in place before the 7 July bombings.