BBC security correspondent Frank Gardner analyses the threat to the UK from a militant network with its roots in Algeria.

The police in Manchester were acting on a tip-off. Where did they get their information from?

Police have been acting on leads that had been generated in some cases by MI5, in some cases by the FBI, the CIA, by the French security service the DST and indeed by the Algerians and other Arab governments.

This was part of a big international operation to try to crack this North African militant network in Europe that is believed to be sympathetic to al-Qaeda.

How do these terror cells show support for al-Qaeda?

It is important to distinguish between logistics operations by al-Qaeda sympathisers - people who are providing stolen credit cards, false passports, money, safe houses, a bit of indoctrination - and the actual active terror cells.

What we saw in Wood Green, in terms of the ricin, looked like an active operation - that is what has really rattled people.

That investigation was like a domino effect - it sparked a lot of other investigations. Leads from that have led to Bournemouth, have led to Manchester, and I am sure we are going to see raids all over Britain in other cities over the next few weeks and months.

Why are Algerian or North African terror suspects living in the UK in the first place?

It is a political hot potato, this. A lot of them have come in as asylum seekers - they have infiltrated into the legitimate Muslim population and hidden themselves amongst ordinary asylum seekers.

Some of these people have become extremely dangerous because they have been subsumed into al-Qaeda's psyche

The reason why a lot of Algerians are here in the first place, why there is a network, is that in 1995 the French anti-terrorist people really cracked down, after a number of bombings in the Metro that were carried out by Algerians that killed eight people and injured well over 100.

Quite a few Algerian extremists sought refuge in the UK.

Britain let them stay here because the authorities didn't see them as a threat to the country, nor did they appear to be breaking any laws.

So they left them alone, and basically ignored them for the first year or two.

The authorities then woke up to the problem, because the French kept insisting these were dangerous people, and started monitoring them.

But they do not have enough resources - they did not then and do not now - to monitor all of them.

Why have they suddenly become a threat to the UK?

In the last two years, some of these people have become extremely dangerous because they have basically been subsumed into al-Qaeda's whole psyche, its whole mindset.

In other words, they have forgotten about their own struggle in Algeria.

But we should not penalise all Algerians or North Africans for this. Most of them are law-abiding people and it is a real shame their name is being besmirched by a very violent minority in their midst.