Paul Wilkinson: 'They are sending a message that they can strike again'

Bombers do make mistakes. In theory they are well-trained and briefed, but they can make mistakes. It is possible they did not wire up the bombs properly, or that the same person made all the bombs and simply got it wrong. It does not mean that they did not intend the bombs to work, I believe that this is very unlikely.

There are a lot of similarities between the attacks yesterday and those on 7 July, with three Tube trains involved and later a bus. They were also similar in method. Certainly enough of a similarity for the police to be investigating as a priority that this was a second wave of attacks that went wrong. Another theory is that this is a more amateur group who were trying to mimic the attacks of a fortnight ago.

The police have the opportunity to gain information that may throw light on the 7 July attacks. However, this assumes that the people who carried out the events are linked, which we cannot be sure about.

We do have to be concerned about further attacks. There is no sign that the terrorist groups are considering giving up attacks in London. Their network in the United Kingdom has not been unravelled, despite progress by police.

It is highly unlikely that the four suicide bombers who attacked the transport network on 7 July were working alone as a one-off operation. They were recruited deliberately and carefully by al-Qa'ida. I suspect that somebody else made the bombs and helped with the logistics of the operation. It is quite possible that there are sleepers, other people ready to be activated. Police are not giving up on the idea that there is a network out there.

Al-Qa'ida has not disappeared from the scene, if anything it has morphed. It is a network of networks, a movement united by an ideology. Terrorist operations are planned and carried out by different parts of the networks.

We should never have thought that European Muslim communities would be immune from penetration by al-Qa'ida. All European countries have al-Qa'ida networks. They are a tiny - but dangerous - minority of Muslims. It does not require huge numbers of people to create attacks - the 11 September attacks in America only involved 19 suicide hijackers.

I am pleased that we are back to a high state of alert because there will be a good level of vigilance from the police.

Though the attempted attacks are worrying, we must be thankful that they did not lead to casualties.

The police and emergency services reacted quickly, endorsing the reputation they gained on 7 July. The reports of calm and co-operation among London travellers are also encouraging and impressive.

Paul Wilkinson is Professor of International Relations at St Andrews University and Chairman of the Advisory Board of the Centre for the Study of Terrorism and Political Violence