Bomb investigation far from over

By Gordon Corera
BBC security correspondent

Tuesday's developments mark a major breakthrough but the investigation into last week's London bombings still has a long way to go.

Police made a breakthrough after searching addresses in Leeds

The key focus will be in trying to track any infrastructure around the four identified bombers.

It is almost inconceivable that the four individuals could have acted by themselves.

A group of apparently unknown, mostly very young men like this, might have the intent to "do something" but having the capability of turning that intent to action would require tapping into broader expertise.

The best evidence for this comes from the explosives used - high grade plastic explosives which are understood not to have been home-made.

These kind of explosives are hard to get hold of in the UK but far easier to get from abroad, in places such as Eastern Europe and the Balkans.

The bombs themselves have also been described as relatively advanced.

It would be a surprise if four amateurs could have got hold of the material and turned it into a bomb themselves.

This points to a possible bomb maker, maybe coming from abroad, as well as the possibility of other organisers - recruiters behind the bombers.

Vital questions

A key focus will be evidence gathered from the bombers' homes.

These four individuals were not on the radar of the security services or police at the time of the attack, providing no warning

As well as providing material for forensics investigation, MI5 will also be looking for lifestyle clues and back history for the individuals.

This is vital in order to try to trace back how they were radicalised and became involved in terrorist activity.

Who did they mix with? Are there individuals who may have drawn them into increasingly militant circles and perhaps activity? Did they travel abroad as has been reported and if so could they have been recruited or tasked with carrying out the bombings whilst there?

All these questions will be pursued as fast as possible.

A further problem comes from the fact that these four individuals were not on the radar of the security services or police at the time of the attack, providing no warning.

It will be important to try to find out if any of them had any contact with known militants or had been in any trouble in the past.

Breaking infrastructure

Individuals are normally picked up if they attend a militant mosque, meet a known militant, seek to buy explosives from known criminal networks or other such activity.

If they did make such contacts there will be questions as to why they were not put on the radar. If they made no contacts it will be even more worrying as a sign of how difficult it is to track and penetrate cells within the UK.

Intelligence is vital but is not easy to come by.

The track record of many investigations into attacks around the world is that it is easier to find the foot soldiers - such as these bombers - rather than the recruiters, the financiers, the organisers and the bomb-makers.

Breaking this broader infrastructure is vital to try to prevent further attacks. There may be other groups of individuals who have the desire and intent to carry out further attacks.

The challenge is preventing them having the capability.