A group of terrorists affiliated with Iran and its Iranian Revolutionary Guard Corps (IRGC) were caught stockpiling tonnes of explosive materials on the outskirts of London in a secret British bomb factory.
A report published by The Telegraph revealed that radicals linked to Hezbollah, the IRGC-affiliated militant group in Lebanon, stashed thousands of disposable ice packs containing ammonium nitrate.
Just months after the UK signed up to the Iran nuclear deal, MI5 and Metropolitan Police forces managed to uncover the plot.
Three metric tonnes of ammonium nitrate was discovered - more than was used in the Oklahoma City bombing that killed 168 people and damaged hundreds of buildings.
Police raided four properties in north-west London - three businesses and a home - and a man in his 40s was arrested on suspicion of plotting terrorism.
The man was eventually released without charge. Well-placed sources said the plot had been disrupted by a covert intelligence operation rather than seeking a prosecution.
The discovery was so serious that David Cameron and Theresa May, then the prime minister and home secretary, were personally briefed on what had been found.
Yet for years the nefarious activity has been kept hidden from the public, including MPs who were debating whether to fully ban Hezbollah, until now.
It raises questions about whether senior UK government figures chose not to reveal the plot in part because they were invested in keeping the Iran nuclear deal afloat.
The disclosure follows a three-month investigation by The Telegraph in which more than 30 current and former officials in Britain, America and Cyprus were approached and court documents were obtained.
One well-placed source described the plot as “proper organised terrorism”, while another said enough explosive materials were stored to do “a lot of damage”.
Ben Wallace, the security minister, said: “The Security Service and police work tirelessly to keep the public safe from a host of national security threats. Necessarily, their efforts and success will often go unseen.”
The Telegraph understands the discovery followed a tip-off from a foreign government. To understand what they were facing, agents from MI5 and officers from Metropolitan Police’s Counter Terrorism Command launched a covert operation.
It became clear, according to well-placed sources, that the UK storage was not in isolation but part of an international Hezbollah plot to lay the groundwork for future attacks.
The group had previously been caught storing ice packs in Thailand. And in 2017, two years after the London bust, a New York Hezbollah member would appear to seek out a foreign ice pack manufacturer.
Why ice packs?
Ice packs provide the perfect cover, according to sources - seemingly harmless and easy to transport. Proving beyond doubt they were purchased for terrorism was tricky.
But the most relevant case was in Cyprus, where a startlingly similar plot had been busted just months before the discovery in London. There, a 26-year-old man called Hussein Bassam Abdallah, a dual Lebanese and Canadian national, was caught caching more than 65,000 ice packs in a basement. During interrogation he admitted to being a member of Hezbollah’s military wing, saying he had once been trained to use an AK47 assault rifle.
Abdallah said the 8.2 tonnes of ammonium nitrate stored was for terrorist attacks. He pleaded guilty and was given a six-year prison sentence in June 2015.
In Abdallah’s luggage police found two photocopies of a forged British passport. Cypriot police say they were not the foreign government agency that tipped Britain off to the London cell.
But they did offer assistance when made aware of the UK case, meeting their British counterparts and sharing reports on what they had uncovered.
MI5's intelligence investigation is understood to have lasted months. The aim was both to disrupt the plot but also get a clearer picture what Hezbollah was up to.
Such investigations can involve everything from eavesdropping on calls to deploying covert sources and trying to turn suspects.
The exact methods used in this case are unknown. Soon conclusions begun to emerge. The plot was at an early stage. It amounted to pre-planning. No target had been selected and no attack was imminent.
Well-placed sources said there was no evidence Britain itself would have been the target. And the ammonium nitrate remained concealed in its ice packs, rather than removed and mixed - a much more advanced and dangerous state. On September 30, the Met made their move.
Officers used search warrants to raid four properties in north-west London - three businesses and one residential address. That same day a man in his 40s was arrested on suspicion of terrorism offences under Section 5 of the Terrorism Act 2006. Neither his name nor his nationality have been disclosed.
His was the only arrest, although sources told The Telegraph at least two people were involved. The man was released on bail. Eventually a decision was taken not to bring charges.
The exact reasons why remain unclear, but it is understood investigators were confident they had disrupted the plot and gained useful information about Hezbollah’s activities in Britain and overseas.
A UK intelligence source said: “MI5 worked independently and closely with international partners to disrupt the threat of malign intent from Iran and its proxies in the UK.”
The decision not to inform the public of the discovery, despite a major debate with Britain’s closest ally America about the success of the Iran nuclear deal, will raise eyebrows.
Keeping MPs in the dark amid a fierce debate about whether to designate the entire of Hezbollah a terrorist group - rather than just its militant wing - will also be questioned.
The US labelled the entire group a terrorist organisation in the 1990s. But in Britain, only its armed wing was banned. The set-up had led senior British counter-terrorism figures to believe there was some form of understanding that Hezbollah would not target the UK directly.
Hezbollah was only added to the banned terrorist group list in its entirety in February 2019 - more than three years after the plot was uncovered.