TREADING stealthily in darkness towards the heart of an international terror plot, two MI5 agents knew this was the most important break-in of their careers.
They had made hundreds of covert entries, but if they were caught sneaking into a nondescript flat in east London thousands of people could die.
The spies were tasked with secretly collecting evidence and planting bugging devices in the property bought by Al-Qaeda terrorists intent on blowing up seven transatlantic passenger planes.
It was a race against time in the summer of 2006, because evidence was mounting that the liquid bomb plot was soon to be carried out.
The death toll was predicted to be even bigger than the 9/11 attacks in the United States five years earlier.
There were 24 suspects in the Al Qaeda network, leading to the biggest counter terrorism surveillance operation in British history.
The intelligence they gained about explosives being squirted into soft drink bottles led to airports banning passengers from taking liquids onto aircraft.
Now 20 years on two of the agents involved in Operation Overt have spoken for the first time about how they caught the wannabe mass murderers in the act.
The covert specialists, one a locksmith, entered into the flat knowing that explosives could be inside and hoping none of the terrorists were still there.
Speaking anonymously in an ITV documentary titled Terror In The Skies: Stopping The Liquid Bomb Plot, MI5’s ‘Dan’ reveals: “It was imperative to get in there and find out what these guys were doing.
“I have worked on hundreds of operations, lots of entries, terrorist properties mainly.
“It feels like the clock is ticking. We are through that door very quickly. If you have been in MI5 long enough you are going to have been involved in operations where people have been killed.
“When the door opens up and you are looking up to the darkness and you are not quite sure if someone is in there, you are thinking ‘God if I f*** this up, this is going to bring to an end probably the biggest counter terrorism operation in the country’s history.’
“There is a degree of pressure. Obviously, there is also a risk of being hurt.”
What they found inside sent shockwaves through the intelligence services both here and in the United States.
There was a row of empty transparent drink bottles, including Lucozade and Oasis, a glass jar full of batteries and another full of gluey liquid.
They returned later, placing listening devices in every room and a camera in the kitchen.
It was vital the spooks left no sign they had ever been there.
Dan explains: “You are not giving the game away. If you go through a door which is ajar, you put it back in exactly the same position.”
On August 3 2006 footage revealed terrorists drilling into the bottom of the bottles and injecting in hydrogen peroxide with an additive to make it appear orange.
The home made devices had a disposable camera for a detonator, which meant they appeared to be a “viable” bomb.
Conversations in the flat also revealed the real names and nicknames of potentially 18 more suspects.
Law enforcement resources from all across the country were called upon to keep track of their movements.
When six of the terrorists were seen recording martyrdom videos it was feared that they were planning to blow themselves up very soon.
That left politicians and senior spooks with a dilemma.
MI5 felt they needed more time to collect evidence in order to prove in court beyond all reasonable doubt this was a genuine terror plot.
John Reid, the then-home secretary, was thinking about raising the terror threat in order to warn the public.
But that risked alerting the plotters that the authorities were on to them.
In the end, Reid followed security service advice and didn’t publish that the threat level was going up.
Keeping the fanatics under surveillance, though, meant something could go horribly wrong.
Reid says: “If as a result of delaying we miscalculated and the conspirators murdered two, three thousand people, my judgement was that no government could have sustained that.
“The government would’ve been in grave danger of falling.”
This was a global operation with officers travelling to South African, Belgium, Mauritius, Japan and Pakistan to gather information.
The stakes rose even higher when one of the lead plotters Abdulla Ahmed Ali was observed looking at flights bound for the United States.
The US President, George W Bush, who was being kept abreast of developments was worried about delaying arrests.
He told one of his team: “Are we on the brink? Let’s not wait too long.”
But it was MI5 who were in charge of this investigation and they wanted to have more proof that the suspects were getting on those planes.
Jonathan Evans, former director general of MI5, says: “There was tension between the UK and the US on this.”
In the end, the Americans forced Britain’s hand.
The CIA received a tip-off that Birmingham born jihadi Rashid Rauf was on the move in Pakistan.
Rauf was believed to be the mastermind of the liquid bomb plot and to have also planned the deadly 7/7 attack on London’s public transport system in 2005.
Not wanting to miss the chance to bring one of Al Qaeda’s most dangerous men into custody, the CIA gave the order to arrest Rauf at a border crossing on August 9 2006.
Having had no warning of this from their US partners, MI5 were left “blindsided” and had to immediately set up an operation room to arrest as many of the potential suicide bombers in Britain as possible.
It was crucial none of them got away.
Later the same day doors were knocked down in London, High Wycombe and Birmingham, with all 24 alleged plotters successfully rounded up.
This was just six days from the footage showing the jihadists preparing the bombs.
That was not enough time for the security services to be certain they had identified everyone involved.
As a result all flights out of the UK were still considered to be at risk of an “imminent” attack.
The day after the arrests passengers had to place all their luggage in the hold and were banned from taking any liquids on board.
Mothers even had to taste the milk in their babies’ bottles to prove it was safe to carry with them.
Later the rules were relaxed to allow 100ml liquids through security and this year seven UK airports increased that allowance to two litres.
Bringing the suspects into custody earlier than planned also had an impact on the court cases.
Defence barristers pointed out that plane tickets had not been purchased and tried to persuade jurors that their clients did not have mass murder on their minds.
Ahmed Ali, who had spoken about bringing his child and wife on the mission, claimed in court that he’d only intended a small explosion at Parliament as a publicity stunt.
But the prosecution had notebooks where some of the plotters laid out their plans in detail, including taking a “dirty mag to distract” security staff.
Only after a retrial in 2009 were Ali, 45, Assad Sarwar, 46, and Tanvir Hussain, 45, found guilty of the plot to blow up the aircraft. Eight other men were convicted of lesser offences.
In total there were seven trials and retrials costing an estimated £70million.
Mohammed Gulzar, 45, who was a friend of Rauf from university and was described by police as the “superintendent” of the operation, was acquitted.
He now lives back in the West Midlands and refused to comment when approached by the documentary makers.
Rauf managed to escape from custody in Pakistan, but was later killed in a US drone strike in 2008 aged 27 at a suspected Al Qaeda base.
What counts is that thousands of innocent lives were saved.
Reid concludes: “This was the biggest counter terrorism operation carried out before or since in Britain.
“There would have been a catastrophic loss of life.”
