By Jon Silverman Home Affairs Analyst

Suddenly, the anti-terrorist stakes have got even higher.

After the dramatic discovery of the lethal poison, ricin, at a flat in North London, the alert against an al-Qaeda attack had moved up a gear.

But the killing of a police constable during an anti-terrorist operation has shocked the security agencies.

The analysis will have to take in several factors.

Their goal is mass casualties in which their own survival is irrelevant

Chiefly, there is the utter unpredictability of some of the Islamic suspects being targeted by the police and MI5.

When the security service took over responsibility for combating republican terrorism, it had a sound base of knowledge on which to build.

As a senior source puts it: "We knew everything about the Provos, which school they had been to, even their dietary habits."

If an active cell was discovered, the security agencies were generally comfortable about building up as much intelligence as possible before deciding when to make arrests.

Mass casualties

The post-11 September picture looks very different.

The Islamic suspects in Britain, many of them from North Africa, are described as "an amorphous target".

It is unlikely that the police expected to find ricin at the Manchester address

The majority are previously unknown to the authorities - though a number have trained in the al-Qaeda camps in Afghanistan.

And their goal is mass casualties in which their own survival is irrelevant so the timing of an attack is virtually impossible to predict.

This explains why one series of arrests has swiftly followed another in the 10 days since the raid on a Wood Green flat and disruption of an alleged ricin plot.

It is unlikely that the police expected to find ricin at the Manchester address.

More probable that the name of the man being targeted had arisen during police investigations into the North London group.

This was the explanation for the arrest of six people in Bournemouth earlier this week.

None has been charged with a terrorist offence but the action sends out a strong message that the security agencies are in an aggressively pro-active mood after being subjected to some, legitimate, criticism that they had under-estimated the threat to the UK, especially from Algerian dissidents.

Pinpointing suspects

It has been said, erroneously, that it was a tip-off from the French which led to the Wood Green arrests.

In fact, intelligence had come in from a number of foreign agencies including some in North Africa.

And information culled, controversially, from some of the detainees at Guantanamo Bay in Cuba, has also been fed back to M15 to help pinpoint suspects in Britain.

For the last year, the government has been under sporadic criticism for its decision to rush through emergency legislation which allows the detention of foreign nationals suspected of terrorist connections, who cannot be deported because of obligations under the European Human Rights Convention.

A number of these people are Algerian.

After the fatal stabbing in Manchester, some of this criticism may be more muted.