By Tom Geoghegan
BBC News Magazine
The government's anti-terrorism plans, which will be discussed by the Lords this week, include a number of proposals to monitor the behaviour of suspected terrorists such as "house arrest". But how would the plans work in practice?
Tony Blair says there are potentially hundreds of terrorist suspects who could be subject to control orders. These are cases where ministers believe the evidence is insufficient or too sensitive to use in a criminal court.
Some people have raised civil liberty concerns about the proposed restrictions, which range from tagging to keeping people under what is effectively house arrest. Others say this is a price worth paying for protecting national security.
Putting that debate aside, how would the proposals work in practice? The Home Office says it is not yet prepared to comment on arrangements for house arrest and the police says it does not discuss any security methods.
But security experts here give their thoughts on how the plans could be implemented.
This would be the most severe of the proposed control orders.
For a start, it would require a premises designed with house arrest in mind, says Charles Shoebridge, a security analyst and former counter-terrorism intelligence officer.
"It's unlikely that a suspect would be detained in his own home. Police will want to ensure that the premises will be one that is effectively secured."
Police would be required to prevent an escape or rescue
That means a detached property with CCTV on all sides and surveillance inside, as well as police officers stationed outside to prevent the suspect leaving or his friends from mounting a rescue attempt, he says.
"However, controlling a suspect's access to communications, even if under house arrest, is problematic. People entering the premises, including children, may have to be thoroughly searched.
"And unless all conversation within the house is monitored, it will be impossible to prevent one occupant passing a message to another - and then on to the outside world."
There are also concerns the house could become a focal point for demonstrations, he adds.
But it need not be so complicated and could work, says John O'Connor, former head of the flying squad.
"Round the clock surveillance by police officers is going to be very labour-intensive and very costly, so I think it will be done electronically. I think mainly it will be a tagging system."
For example, this could mean the police being alerted at a control centre if the tagged suspect left the house without a police escort. Such a system would require close surveillance throughout the house and the police monitoring it, at a cost of about £240,000 a year, between the police and the Home Office.
More high-risk suspects would need an armed police presence standing by or outside the premises, at a total cost of more than £1m a year, Mr O'Connor estimates.
The satellite technology for this is already in use for offenders on early release from prison.
And a version which pinpoints the location of someone wearing a tag to about two metres is being piloted in parts of the UK.
Offenders wear an electronic ankle tag with a wireless connection to a belt-worn device that transmits co-ordinates to the tracking system. An alarm is triggered if this device is separated from the anklet.
A curfew is enforced by the same principle, only the timing of the breach would determine the alarm.
But Mr Shoebridge doubts whether tagging would be effective. "Electronic tagging has a role to play in controlling burglars, for example," he says.
"But in the case of a suspect considered so dangerous that he is subject to an anti-terrorist control order, tagging will do little to prevent an attack. For those prepared to commit suicide for their cause, the threat of prison for removing a tag will be no deterrent at all."
Tagging is certainly a lot cheaper than the other options. In launching the pilot using the latest technology last September, the Home Office estimated it could average £68 per person per day (£24,820 per year).
MOBILE PHONE & INTERNET BAN
This isn't too difficult but it requires police manpower to follow the suspect when he leaves the house, says Mr O'Connor. The premises will already have been cleared of phone and internet connections, and he won't be allowed to leave the house without a police officer.
But Mr Shoebridge thinks enforcing a ban on communications is likely to divert considerable surveillance resources from other anti-terrorist operations.
"Take for example a suspect who speaks to one person, who then speaks to another, who then accesses the internet on the suspect's behalf. All who came into contact with all three would have to be under 24 hour surveillance - and this is simply not going to happen."
The fact the suspect and his contacts know he is under a control order means surveillance, though necessary, is unlikely to produce much of intelligence value in any case, he adds.
But how easy is it using technology alone?
Phil Robinson, chief technology officer for security firm IRM, says there is kit which could be installed at a premises to monitor mobile phone emissions, but someone walking past outside could set it off.
Internet access is trickier because there are so many ways to do it, such as mobiles, Bluetooth, 3G, ADSL, cable and dial-up, but there is technology which could be used to make sure these options are closed off in any particular house.
Both kits, for phone and internet, cost tens of thousands of pounds, so Mr Robinson thinks a visit in person may be a more realistic method.
1: Offender wears tracker device on belt, with wireless connection to ankle tag.
2: Belt device tracks own location using GPS satellite signals.
3: Location data transferred from belt device to telephone network.
4: Data sent to control centre, which can be specifically alerted if offender enters exclusion zone or breaks other conditions. Radio frequency alarm also sent if tracker and ankle tag are separated.
5: Offender's position shown as location trails on computer map.