Police risked offending delicate religious sensitivities by raiding the Finsbury Park mosque, but their actions were justified by the mini-arsenal of weapons, terrorist paraphernalia and forged passports they found inside.
Operation Mermant, which began in the early hours of January 20 2003, involved scores of officers in body armour using battering rams to enter the building.
Full details of what they discovered during three days of searches can only be revealed today following the conclusion of Abu Hamza's trial on race hate charges.
The stash of equipment included chemical warfare protection suits, or NBC (nuclear, biological and chemical) suits, as they are technically known.
They were found together with three blank-firing pistols, a stun gun and CS spray.
Officers also found a gas mask, handcuffs, hunting knives and a walkie talkie.
Detectives believe the equipment was being used in terror training camps located somewhere within the UK.
It is not clear exactly where these were, but speculation in the past has centred around remote parts of Wales, in particular the Brecon Beacons, and national parks such as those in the Highlands, Yorkshire Dales or Lake District.
"Our assessment was that this was material that had been used in training camps, probably here in the UK," a senior police source said.
Some of the material was found close to Hamza's office, although police sources admit they could not put his fingerprints on it, "literally or metaphorically".
Police also found more than 100 stolen or forged passports and identity documents, laminating equipment, credit cards and chequebooks hidden under rugs and concealed above ceilings. One officer recalled pulling down part of a ceiling to find passports raining down on him.
The raid followed the discovery of what was then thought to be ricin at a flat in nearby Wood Green during a swoop earlier that month, on January 5.
The material found there went on to form the core of the trial of al Qaida suspect Kamel Bourgass and the others accused of involvement in an alleged plot to manufacture the deadly poison.
A senior police source said officers were led to Finsbury Park by a "clear evidential lead" discovered during the Wood Green raid, which left police with "no option" but to search the mosque.
This is believed to be an envelope, which had the address of the mosque on the front, in which police found recipes for chemicals including ricin and cyanide.
It was a clear sign that the mosque, under Hamza's stewardship, was more than just a talking shop.
A week after the Wood Green raid, the owner of the recipes, Bourgass, or Nadir Habra as he was also known, was arrested at a flat in Manchester.
He stabbed to death Detective Constable Stephen Oake and seriously injured three other police officers as he tried to escape.
Bourgass, like other illegal immigrants who needed a place to stay at that time, had been dossing down at the mosque in the weeks before his attempts to make ricin were discovered.
He had used it as his postal address and it was the envelope in which the Immigration Service sent details of his refusal for asylum that he later used to store the poison recipes. A photocopier in the mosque was also used to copy his original hand-written recipes.
A week after the arrest of Bourgass, and after much agonising at the highest levels of Scotland Yard and the Home Office, detectives decided they had to take the controversial step of raiding Finsbury Park mosque.
Operation Mermant was launched in the early hours. Illuminated by a beam from a helicopter, dozens of officers in body armour used an "enforcer" battering ram to get in before donning overshoes as a sign of respect.
Police had been unable to release full details of the material found during the raid before today because of the risk of prejudice to Hamza's trial.
But officers said it was important to do so now to "show why it was we felt it necessary to take the extreme step of searching a mosque".
"We recognised how sensitive it would be and took every precaution to make sure as far as we could that we did not act in an insensitive manner," a police source said.
"We worked with the original trustees, who had been ousted by Abu Hamza and his followers."
Muslim officers also accompanied police on the raid to advise on appropriate behaviour. They were appalled at the state of the mosque.
"They were shocked at the condition of the mosque, the lack of cleanliness and care," the police source said.
Hamza condemned the raid, accusing the police and Government of adopting "Rambo-style" tactics, describing it as "provocative", "silly" and "illogical". Other Muslim leaders also criticised the manner of the operation.
It has been alleged that Hamza rose to prominence at the mosque by bullying and intimidating the trustees, driving away moderate Muslims.
He provoked outrage with his speeches supporting al Qaida leader Osama bin Laden and the September 11 attacks. He even once described the American shuttle disaster as a "punishment from Allah".
Following an investigation by the Charity Commission, the cleric was suspended from his post there in April 2002, but was not formally dismissed until February the following year. The commission concluded that Hamza had used it for "personal and political, rather than charitable, purposes".
The mosque, which is officially known as the North London Central Mosque, was closed and boarded up for several months following the police raid.
After a period of refurbishment, it reopened in February last year with a new board of trustees, heralding a fresh start.
They claimed to have removed the "minority of radical elements" who had taken unwanted control of the mosque and "tainted its reputation in the eyes of the public at large".
"The previous incumbents had preached an ideal of Islam that is not condoned or supported by the mosque's current trustees, or the Muslim community who currently use the mosque," a spokesman for the mosque said last year.
Police say they have now built a "strong working partnership" with all involved at the mosque and that officers are now regular, welcome visitors.
A senior police source said it had been "helpful" to the community in the Finsbury Park area to have Hamza removed from his role there, so that people could go back to worshipping at the mosque in "an atmosphere of calm and tranquillity".
"As opposed to what was happening before, when he and his followers were creating a degree of conflict," the source said.