Bomb-making equipment has been discovered by anti-terrorist officers investigating the foiled plot to blow up 10 transatlantic airliners, intelligence sources say.
MI5 and the police believe a Britain-based terrorist cell, assisted by al-Qaeda members, had been planning to start a series of suicide bombings on American-bound planes as early as yesterday or today. It has also emerged that there was a police informer working closely with the plotters.
The alleged terrorists were foiled after police carried out the series of raids in which they arrested 24 people, mostly young British men of Pakistani descent, in east London, Buckinghamshire and Birmingham.
The police were rushed into making the arrests after one of the alleged ringleaders - a British citizen - was arrested in Pakistan on Wednesday, US intelligence sources have disclosed.
The police acted swiftly because they were fearful that the arrest in Pakistan would alert British based suicide bombers and prompt them into carrying out the planned attacks in the next few days, the US sources confirmed.
Anti-terrorist officers are understood to have found material and documents that could be used to make liquid explosive bombs for smuggling onto aircraft, at houses in east London and High Wycombe in Buckinghamshire. There are also unconfirmed reports of one or two "martyrdom videos" - recordings made by would-be suicide bombers - being discovered.
The suspects are accused of planning to use specially adapted sports drinks bottles to smuggle in the explosive material, which could be detonated with a battery, or flash from a disposable camera.
An unprecedented year-long surveillance operation by MI5 and the police is alleged to have uncovered a plot to blow up nine or ten airliners, killing up to 3,000 passengers and crew, in three phases.
The investigation has discovered a series of links with Pakistan, where several of the alleged plotters are thought to have been partly trained, financed and radicalised by al-Qa'ida members, although the alleged plot remains largely a homegrown affair, intelligence officers believe. The alleged plan was to use a homemade explosive and smuggle it through airport security in hand luggage.
Three of four suicide bombers would each board a passenger plane heading for the United States and detonate the devices at the same time. Two further waves of attacks would then be carried out at later dates.
The terrorists had got as far as identifying several American-owned airlines to target, but had yet to select specific flights or buy any tickets, it is understood. Security sources believe they intended to purchase tickets at the last minute.
The alleged plot has caused chaos to airports and forced the authorities to introduce strict new security measures, including banning hand luggage.
Britain also remains on the highest level of security - known as critical - although the Home Office has admitted this is only as a precaution. Police continued to search more than 20 properties yesterday and are preparing to question the 24 suspects being held on suspicion of the commission, preparation or instigation of acts of terrorism. Nineteen of the suspects have had their bank accounts and assets frozen.
Several of the people arrested, which included a mother and her child, are expected to be released without charge in the next few days.
As the investigation progresses more details of the the operation have emerged.
Known as Operation Overt, the US network, ABC News, revealed that the British police had penetrated the alleged cell and had someone working for them on the inside.
MI5 were reportedly alerted to one of the suspects after a relative became suspicious and contacted the authorities, according to an unconfirmed US report.
It was also disclosed that the reason the police had to bring forward their plans by about a week was that one of the leading alleged plotters was mistakenly arrested in Pakistan.
The detention on Wednesday of a man named by the Pakistan Foreign Ministry as Rashid Rauf, a British national, caused alarm among the British police and intelligence agencies because they feared that his arrest would scare his alleged conspirators into bringing forward their attack plans.
Police have said that they have arrested all the suspected main players, although it is not clear whether everyone involved in Pakistan has been detained. But John Reid, the Home Secretary, said: "We can never be certain and we want to be sure that, alongside the operational interventions we made, we maintain a very high level of vigilance and the necessary restrictions on the aviation sector."
Airline chiefs yesterday accepted airport restrictions would have to be tightened permanently, following a meeting with Cabinet Ministers.Reuse content