Home Secretary David Blunkett says he wants to tackle terror attacks before they happen, and is proposing new laws which he says will help do it. But the idea of predicting who might commit crimes is an extremely tricky area.
By Brian Wheeler and Jonathan Duffy
BBC News Online Magazine
Mr Blunkett's plans, revealed this week on a six-day trip to India and Pakistan, include keeping sensitive evidence from defendants and secret trials before specially-vetted judges.
He also discussed lowering the standard of proof required by a court to secure a conviction.
The Home Office has said it will be releasing a paper outlining options for the renewal of the 2001 Anti-Terrorism, Crime and Security Bill next month.
It refused to comment on whether that would include "pre-emptive" trials, of the type described by Mr Blunkett.
But it said the overall aim of the legislation would be to tackle terror attacks before they happen.
Mr Blunkett says he is determined to avoid an 11 September-style atrocity occurring on British soil.
But the idea of pre-emptive justice - for instance locking people up before they have committed a crime - is fraught with danger.
It is, civil liberties groups say, one of the hallmarks of repressive states throughout history.
For some, it contains uncomfortable echoes of George Orwell's "thought crimes". For others, Minority Report.
But there is also a question mark over how effective it would be at stopping a determined suicide bomber.
"The accent needs to be on prediction to tackle this sort of crime, but as regards its effectiveness so far, it's unclear," says Michael Levi, professor of criminology at Cardiff University.
"You could argue that if the intelligence about weapons of mass destruction was so wrong why should you accept its credibility in this realm."
The doctrine of "pre-emption" has been quietly gaining ground in other areas of crime fighting, most notably in the field of sex offences and internet paedophilia.
But the most widespread application is probably in the drugs field, where HM Customs for many years have used sophisticated techniques to determine which passengers on a flight might be carrying drugs.
This has made drugs policing more effective, but has not prevented huge quantities of drugs from getting through.
"In tackling crime, an awful lot of effort has been put into so-called 'preventative risk management', be it anti-money laundering, civil restraint measures, asset recovery," says Professor Levi.
He said it was part of "a general move away from relying on the criminal justice system".
Alex Standish, editor of Jane's Intelligence Digest, says the problem is that the authorities are unable to gather evidence of sufficient quality to convict suspected terrorists under the current system.
Mr Blunkett: Determined to prevent attack
"The intelligence services don't have a particularly good handle on what is happening within radical Islamic groups in this country.
"One of the reasons is that it has proved extraordinarily difficult to recruit people who can be security cleared."
But that does not mean the rules should be changed to make it easier to secure convictions, he says.
"Pre-emptive" trials, he argues, contain echoes of internment, the policy of imprisonment without trial used in Northern Ireland in the early 1970s.
Internment - and so-called Diplock courts that sat without juries - caused widespread resentment and were abandoned.
"We have done all this before. It didn't work then and it won't work now, in terms of practical policing. I think in a sense it is posturing, rather than looking at serious policy", Mr Standish adds.
The government should be concentrating instead on recruiting more intelligence officers from ethnic minority communities, he says.
This is already happening to a certain extent, according to security analyst Glenmore Trenear-Harvey.
"MI5 has in the last year one of the most successful periods of recruitment, working with careers officers they've reduced their cloak of secrecy and been more practical.
"In Leeds, Bradford and Newcastle they have been recruiting from a broader ethnic basis, particularly looking for agents with language skills.
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"It's not so much someone walking into the local nick saying I've got a suspicion. It's a case of MI5 running agents in the community and reporting back," says Mr Glenmore Trenear-Harvey.
The security services are also making more use of "profiling" techniques, such as tracking someone who has made several trips to the Asian sub-continent in 12 months, Mr Trenear-Harvey says.
But a determined terrorist will always be able to stay one step ahead of the authorities by maintaining a low profile.
"Al Qaeda has some of the best IT and financial brains and will keep them as sleepers, these seemingly honourable and respectable people," says Mr Trenear-Harvey.
Profiling will not identify these people, Mr Trenear-Harvey says, but "will pick out the foot soldiers, like the shoe bomber Richard Reid".
He believes the civil liberties backlash against such measures is due, in part, to the government failing to properly explain the consequences of a terrorist attack.
The public was "still in an IRA mindset in that those terrorists still wanted to live".
"Suicide bombers consider themselves expendable and in modern times we've never had to contend with that."
But the biggest hurdle in the way of Mr Blunkett's proposals is likely to come from within his own party.
One senior Labour MP told BBC News Online "a fairly large block" in the party would be against Mr Blunkett's proposals as they stood and would fight to prevent them becoming law.