1 June 2013 Last updated at 23:20 GMT
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Terror watch lists: Can you keep tabs on every suspect?By Ruth Alexander BBC
News
After the Boston Marathon bombing and the killing of a British soldier on the
streets of Woolwich in London, it emerged that suspects were known to
the security services - prompting concern from critics. But how feasible is it
for the spies to monitor everyone on their watch list?
Michael Adebolajo and Michael Adebowale's names have become
well known since they became key suspects in the killing of Drummer Lee
Rigby in a street attack in Woolwich on 22 May.
But it has transpired that they were already familiar names
to the British domestic intelligence service, MI5. The UK Parliament's
Intelligence and Security Committee is to investigate the agency's
actions in relation to the case.
Boston bomb suspect Tamerlan Tsarnaev, meanwhile, was questioned in 2011 by the
FBI amid claims he had adopted radical Islam.
But is it practical - or even possible - to keep close tabs on every person who
comes to the attention of the security services?
No, according to Dame Stella Rimington, former head of the of MI5. To see that,
she says, you just have to look at the numbers.
It's not known how many people are on the terrorism watch list in the UK. But
it has been said to be around 2,000.
To keep a constant watch on just
one of those people, you would need a team of at least six surveillance
operatives, Dame Stella says. But of course they couldn't work 24 hours a day,
so you would need three teams of six.
And those operatives couldn't just sit outside a suspect's
house. So, you'd need an additional person to, say, sit in a nearby
house, and alert the team of six when the suspect left the house.
Then there's the control centre, where staff receive
information from the mobile operatives and give them directions. And
finally, there's a desk officer in charge of the case.
"Doing that 24 hours a day, seven days a week - well, you do the sums, it's an
awful lot of people," Dame Stella says.
And if 2,000 people were to be followed like that, we'd be
talking about 50,000 full-time spies doing nothing but following
suspected terrorists. That's more than 10 times the number of people
employed by MI5. The numbers don't add up.
Boston bomb suspect Tamerlan Tsarnaev had been questioned by authorities
As a matter of simple economics, then, it's not possible to
follow every suspicious character around the clock. Surveillance
technology offers an alternative - but it also presents mathematical
problems of a different kind.
Imagine that the intelligence services had unlimited resources and could
monitor everyone's phone lines.
Imagine they could detect would-be terrorists within the
first three words they utter on the phone with a 99% degree of accuracy.
There would just be one small problem, according to Howard
Wainer, a professor of statistics at the Wharton School of the
University of Pennyslvannia in the United States.
Before they were Tamerlan and Dzhokhar Tsarnaev, they were
Black Hat and White Hat - two young faces in baseball caps, identified
in grainy video footage as the prime suspects in the Boston Marathon
bombings.
Those bombs would kill three people and wound more than 200,
and Black Hat and White Hat were identified soon after their images were
released. A day later, one was dead and the other in hospital.
The crucial role that video footage played in this case has
many Americans re-evaluating the role of CCTV and other surveillance
tools in public spaces.
* Read the full article from April 2013 here
Suppose there are 3,000
terrorists in the United States, he says. If the software is 99%
accurate, you would be able to pick up almost all of them - 99% of them.
However if you were listening to everybody - all 300 million US
citizens - 1% of the general population are going to be picked up by
mistake.
"So mixed in with the 3,000 true terrorists that you've
identified are going to be the three million completely innocent people, who
are now being sent off to Guantanamo Bay," Wainer says.
That is, for every terrorist you would have 999 innocent, but very angry people.
In reality, Wainer says, your terrorist detector would be
nowhere near 99% effective. But, on the other hand, the security
services do not in fact monitor everyone.
Still, if you narrow your target population to the point that there is one
actual terrorist per 100 people wiretapped, and assume a
90% effective test, the chance of a false positive remains high. Even
when someone triggers an arrest, Howard Wainer says, the odds are 11 to
one that they're not a terrorist.
It's hard to predict who will become radicalised, and who
will go on to commit an act of violence, according to Nigel Inkster, who used
to work for MI6, the UK's secret intelligence service abroad and
is now employed by the International Institute of Strategic Studies, a
London think tank.
"With the wisdom of hindsight, everything always looks clear, but when you are
looking at the situations as they unfold, you are
operating in a climate of considerable uncertainty," he says.
Continue reading the main story
“Start Quote
Intelligence services can strangle themselves if they have too much
information”Dame Stella Rimington Former head of MI5
Technological advances mean
intelligence services can use computers to sort through lots of
information to help target the right people.
Algorithms are used to search social media for key words, or
to spot suspicious patterns in airline travel records, for example.
But these techniques may be of limited use, according to
Louise Amoore, a professor at Durham University specialising in data and
security.
"You may already begin to think about how the algorithms used to detect
possible risky connections might be adapting," she says.
"For example, post-Boston there may be more attention in the
US to travel to particular parts of the world, perhaps including
Chechnya and Dagestan. We could imagine, post-Woolwich, that there might be
greater attention in the refining of algorithms to think about
patterns of travel and links to deportation.
"But of course, it's using data from past events. Our
research is suggesting that the tuning of the algorithm reflects almost
always past events."
In other words, the algorithms are always fighting the last war.
And, as for the useful information that can be gleaned from the data, there is
a risk it can be lost in the noise.
Big data is one of the challenges that security and
intelligence organisations around the world now face, according to Nigel
Inkster.
Continue reading the main story
More on Woolwich and Boston attacks
* Woolwich attack: Full details
* Boston bombings: Full details
"You're able to amass large
quantities of data, beyond what is possible for one individual or group
of individuals quickly to analyse and assimilate," he says. "Algorithmic
approaches are being adopted to try and triage this information, but I
don't think you are ever going to be able to develop algorithms which
can substitute for or improve upon human judgement."
Dame Stella points out that this is a well-known problem - it happened to the
East German Stasi. They "overdosed" on information, she says.
"Intelligence services can strangle themselves if they have
too much information, because they can't sort out from it what they need to
know and what they don't need to know. So in all this search for
information you've got to be pretty focused and targeted."
[Non-text portions of this message have been removed]
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