The biggest surveillance operation in British history is under way this week as the authorities seek to track what the Home Secretary, John Reid, has called "dozens" of terror plots. While every police force in the country is now involved in investigating the alleged plot to bring down transatlantic airliners, MI5, the main counter-terror agency, is being strained to the limit as it seeks to head off the next threat.
"The one thing we can be sure of is that there will be a 'next one'," said a Whitehall source. "The big question, indeed, is 'Where next?'"
Ten days after the world learned about Operation Overt - the wave of arrests in this country and Pakistan which is said to have prevented the attacks on air travel - airports are returning to something approaching normal. Most flights are getting away, and the initial prohibition on virtually any hand baggage has been eased.
The 23 suspects arrested in the early hours of 10 August in High Wycombe, Birmingham and east London are being held in high-security police stations, with few details emerging about their interrogations. A further 17 people - some British, some local citizens - are reported to have been arrested in Pakistan, where intelligence officials fuel speculation about al-Qa'ida links and "masterminds".
Here, although the Home Secretary has spoken of the suspected "main players" being accounted for, and "substantial" material having been found at suspect properties, few other details have been forthcoming. Unofficial police leaks of the discovery of "martyrdom" videos, firearms and a bomb-making kit have not been confirmed.
The national threat assessment, raised to "critical" - its highest level - when Operation Overt was launched, has been lowered one notch to "severe". But behind the scenes there is acute concern and intense activity.
According to security service estimates, there are about 400 potential terrorists in Britain, including a "hard core" of around 50 people, many of whom have undergone advanced training in terror camps and aim to carry out attacks here. Many are being watched day and night amid fears that at least one, possibly two, other significant plots are well advanced, and may be brought forward in the wake of Operation Overt.
MI5 has borrowed surveillance teams both from MI6, the foreign intelligence service, and the new Serious Organised Crime Agency in the race to stay ahead of would-be terrorists. "Everybody who has ever been trained in surveillance is being recruited in to help," said an intelligence source. "It is clear that MI5 are very, very worried about someone attempting another major attack."
Beyond those of immediate concern, there may be as many as 800 more extreme Islamist "peripherals" who could become active terrorists at any point, giving the security services further problems in deciding who to watch. MI5 has admitted it failed to follow up evidence that the ringleader of the London bombings last year, Mohammed Siddique Khan, was on the fringes of an earlier terrorist plot before going on to lead the 7/7 attacks.
Most surveillance teams consist of 16 people, but it can take up to twice as many to trail someone who is very active and does not follow routines. It is believed that about 80 suspects are under intense surveillance, requiring hundreds of officers working full time. Others are monitored more lightly, but still absorb considerable manpower.
"The number of people you need to follow someone can depend on how serious a threat a target may be," said Jim Smith, a former Scotland Yard detective and surveillance expert. "How they travel is the first thing you find out: do they walk, use public transport or a car? If they walk a lot, that can be even more difficult than if they use a car.
"In my experience, a team needs several people on foot, a car and a motorbike. A motorbike is vital if you are following a car, especially in towns, because it is anonymous, and can hang back in traffic and then catch up quickly, getting through traffic lights if they are about to change.
"The important thing is to work out where you can pick up the target you want to follow. You want to be as far as way from their home as possible. The first half mile is the most difficult because targets are more suspicious of being followed from their home."
A former security officer said: "It's clear that many of those suspected of being involved in extremist activity have knowledge of some of the techniques that can be used against them and have become more savvy of surveillance activities."
Terrorists well know that the best time to move can be at night. This is not only because surveillance teams may assume they are sleeping and lower their guard, but because following someone on dark, deserted streets is much more obvious than during the day. Mounting surveillance in areas with a large ethnic minority population can be tricky for MI5, which still tends to attract mainly white recruits. Terror suspects also tend to live where residents are suspicious of the police or anyone else who does not appear local.
Modern technology, such as mobile phones, computers and emails, can help terrorists as well as those seeking to thwart them. A suspect's mobile will be monitored by the signals intelligence agency, GCHQ, whose experts carry out "traffic analysis" of all calls to and from the phone, building up a picture of his contacts and, where appropriate, seeking fresh warrants to monitor their phones. As long as the target's mobile phone is switched on and he has it with him, it can be used to listen to anything he is saying to anyone else.
Tensions have arisen in the struggle to secure evidence of the "airliners plot" and prevent future attacks, both among agencies in this country and between Britain and other countries, notably the US and Pakistan.
American pressure triggered the arrest in Pakistan of Rashid Rauf, the former resident of Birmingham accused of a key role in the plot.
"Britain wanted the Pakistanis to keep him under observation and help gather evidence against him, but they and the Americans have far less concern for due process," said one source. "But when Pakistan arrests top suspects wanted by the Americans, they are on a plane out of the country within hours, and disappear into a secret CIA prison. There is no mention of a trial."
Both Britain and Pakistan say the question of Mr Rauf's possible extradition is some way off, but yesterday there were claims that he might have Pakistani as well as British citizenship, further complicating the issue. Pakistan's main concern throughout has been to play down local links to al-Qa'ida; a stream of leaks from local intelligence sources have sought to stress that the alleged plot was supported from Afghanistan, with a new "mastermind" being suggested almost every day.
Following Mr Rauf's detention, British sources say, they observed a surge in electronic traffic from Pakistan to the UK, suggesting the conspirators were speeding up their plot. This forced the authorities to move in much sooner than they wished, which is why the investigation is at such an embryonic stage.
According to another official, the US administration wanted to issue a statement about Operation Overt before the arrests here had even been completed. In the first few days, most of the detail about the alleged plot - far more than Britain apparently wanted to disclose - came from US Homeland Security officials.
According to one police source, some senior officers were angry that they had to carry out the arrests so early in their investigations.
There is concern in Whitehall over police leaks about evidence, such as the "martyrdom" videos and bomb-making materials, which have allegedly been found. Suspicions are growing that the Metropolitan Police wants to exploit this case after the embarrassing errors in July's Forest Gate raid, when one innocent man was shot and no evidence of any terror plot was found.
"The Met are pursuing this as a public relations exercise after Forest Gate, at the risk of jeopardising everything," said one source.
TEN DAYS ON: WHAT WE KNOW AND WHAT WE DON'T
Ten days after the world learned that a conspiracy "intended to be mass murder on an unimaginable scale" had been disrupted, much remains to be disclosed.
WHAT IS KNOWN
Government sources say there was a plan to bring down as many as a dozen airliners flying to the US, possibly in "waves" of two or three at a time. It is alleged that two apparently harmless chemicals would have been combined in flight to produce an explosive, and detonated by an electronic device such as an iPod or a camera. The Home Secretary, John Reid, has said the main suspects are in custody, and that there is "substantial" material evidence to support the case.
WHAT IS NOT KNOWN
There are major question marks over how close the alleged plan was to fruition, and the intended timing of any attacks. It is claimed that TATP, the explosive used in the London bombings last year, would have been used, but the process for producing it is risky, difficult and almost impossible to carry out unobtrusively. Experts have been at a loss to explain how it would have been possible to create this explosive in an aircraft toilet.
WHAT IS KNOWN
Britain continues to hold 23 suspects in custody, all of them arrested in the early hours of 10 August in High Wycombe, Birmingham and east London.
At least two are said to be women. A 24th person arrested later was released within a day. Police will have to gain court agreement tomorrow to extend the detention of two of the suspects, with renewal due for the other 21 on Wednesday. Pakistan is reported to be holding another 17 people, including Britons, but Rashid Rauf, whose family moved to Birmingham after he was born in Pakistan, is the only one to be named, and fresh doubts have been raised about what citizenship he holds.
WHAT IS NOT KNOWN
Although the Bank of England named 19 of the suspects when freezing their assets, including Tayib Rauf, the brother of Rashid, the identities of some remain unknown. Most reports about those arrested in Pakistan are unsourced and speculative - yesterday there was a claim that the Raufs' father, Abdul, had been held in Islamabad as he went to catch a flight back to Britain.
Links to other plots
WHAT IS KNOWN
Police and security services are investigating any possible connection to the 7/7 and 9/11 attacks, but no conclusive links have emerged. It is believed, however, that the alleged orchestrators behind this latest thwarted plot do share a connection with one of the London suicide bombers, Shehzad Tanweer. Like him, they have been associated with Lashkar-e Taiba, a radical Islamist group based in Pakistan. This Kashmiri militant group ran the madrasa near Lahore which Tanweer visited a few months before the July 2005 attacks. Hafiz Mohammed Saeed, said to be the founder of Lashkar-e Taiba, is currently under house arrest. It has also been alleged, though not officially confirmed, that one of two British-born men arrested earlier this month in Pakistan left a voicemail message last year which was found on a phone in the home of one of the 7/7 bombers.
WHAT IS NOT KNOWN
There is speculation that the current plot was planned to coincide with the fifth anniversary next month of the 9/11 attacks. However, there is no direct evidence to support the theory, and intelligence sources have indicated that the attacks were in fact scheduled to take place last week, not in September. Among other unanswered questions are whether the alleged plotters were recruited by the same figure or figures as those behind other plots, and whether there is any common link between the types of explosives involved.
The Pakistan connection
WHAT WE KNOW
Rashid Rauf moved to Pakistan four years ago, shortly after his uncle was murdered in Birmingham. After initial claims that he was the mastermind of the alleged plot, Pakistani officials now appear to be playing down his significance. This may be because his most obvious connection is not to al-Qa'ida, but to a Pakistani militant group, Jaish e-Mohammed. His wife is the sister-in-law of Jaish's chief, Maulana Masood Azhar.
WHAT IS NOT KNOWN
Although most of the suspects arrested in Britain are of Pakistani origin - The Independent on Sunday has learnt that some may not be British citizens - little has emerged about how many had travelled to Pakistan, how often, how much time they spent there or whether they had received any training. Accounts of where and when Rashid Rauf was arrested, the event said to have triggered the security swoop here, vary wildly.
The al-Qa'ida connection
WHAT IS KNOWN
British security sources have been confident from the outset that this plot is connected to al-Qa'ida, unlike some others where a link remains uncertain. There have been allegations that one of those held is al-Qa'ida's chief in Britain, and that two or three of the others were connected to the network, but official sources have refused to make any comment on these claims.
WHAT IS NOT KNOWN
While agreeing that there is a link to the al-Qa'ida network of Osama bin Laden (pictured), Pakistan has done its best to insist that the trail runs through Afghanistan. The campaign began with reports that Rashid Rauf was held near the Afghan border, when it is much more likely that he was picked up where he lived, in the south Punjab town of Bahawalpur. It continued with almost daily suggestions of different masterminds, all conveniently said to be in Afghanistan. These included an unnamed figure said to be al-Qa'ida's third in command, an unspecified son in law of the deputy leader, Ayman al-Zawahiri, and Zawahiri himself. It is far more plausible, however, that the link may be a Pakistani, Matiur Rehman, who also lived in Bahawalpur and may or may not have been arrested.Reuse content