BBC News Online looks at the details.

How long has the investigation in Leicestershire been going on?

There has obviously been a big investigation going on in the area since shortly after the 11 September attacks in the United States.

Do police suspect there is an al-Qaeda network in the area?

Only last week Scotland Yard said they thought there might be 100 al-Qaeda suspects in the UK and clearly there is some kind of suspected network in the Midlands.

How significant is it that the two men who have been charged with involvement with the al-Qaeda are from Algeria?

A couple of weeks after the 11 September attacks two men were arrested in Leicester.

One of those was very significant, an Algerian man who was subsequently extradited to France and charged with complicity in an al-Qaeda bombing campaign on the continent.

Since then there have been a lot of investigations going on involving Algerians in the UK.

Are the men thought to be linked to the 11 September attacks?

We know that they are thought to be part of a pan-European terrorist network, but it is clear that they are not being charged in connection, or are thought to be connected in any way, with the 11 September attacks.

The suspicion is that these arrests are as a direct result of the arrests in September.

What other arrests have been made?

As well as the nine people who have been arrested under the Terrorism Act 2000, eight have been arrested under the Immigration Act 1971.

Has anyone else been involved in this investigation?

The Metropolitan Police anti-terrorist branch has a national responsibility. Every time there is a local investigation by one of the police forces outside London their expertise is called upon.

There has also almost certainly been input into this from the security service MI5.