OPINION is divided among those investigating whether the two groups of bombers who attacked London’s transport network are linked.

One senior officer close to the investigation told The Times: “For every striking similarity we find in the targets, the planning and the forensics of the two groups, we then uncover a major difference between the two operations.

“Is it conceivable that those allegedly involved in the July 21 incident woke up on the day after 7/7 and thought, ‘Let’s do the same’? But if they are part of the same cell, then why were the consequences of the two attacks so different?”

Police and security services admit they are having to divide their time between investigating the July 7 attack which killed 56 people and the botched bombings a fortnight later, as well as trying to identify a possible third cell.

“The priority has to be stopping another attack, but in the past week we had to find and arrest the July 21 suspects and their support network, and so we could not devote as much manpower to investigating the July 7 operation as we would like,” the officer said.

It was the indiscreet New York police chief, Raymond Kelly, who this week revealed to an audience of Wall Street bankers and Manhattan store owners the secrets of Scotland Yard’s forensic comparison between the two groups. The Yard has made plain its displeasure at the leaks.

Mr Kelly revealed how of all the types of explosives the two groups could have employed, both appeared to choose roughly the same formula. There are scores of instructions on the internet on building bombs but both groups seem to have gone for a home-made peroxide based mix. Ingredients could be obtained easily at hardware stores and beauty parlours, and the mix was apparently brewed up in flats being used by some of those who allegedly took part in the attacks.

However, the sophistication of the first batch of rucksack devices suggests a more proficient bombmaker than for the July 21 operation. Nobody in the investigation takes seriously Hussain Osman’s claims in Rome that the 21/7 attack was intended to scare, not maim.The usual practice for Islamic militant groups has been to keep their cells separate, with only a handful of figures knowing the identity of both teams.

In Britain the investigators have yet to identify any sort of mastermind.

There is still no hard evidence on who first engineered the plot to paralyse London’s transport network, let alone who talent spotted, radicalised and recruited the nine alleged bombers. Several people have been arrested and charged as part of the 21/7 inquiry, but nobody is being held over the first wave of strikes.

The intelligence agencies admit they knew nothing of either cell before the attacks. Subsequent inquiries have shown that a number of the alleged bombers did have ties with terror suspects, mostly abroad. There are times and places where the paths of both groups cross, including a radical London mosque, but otherwise these men appear to have lived separate lives.

Some of these inconsistencies may be resolved next week when Scotland Yard detectives will be allowed to speak to Hussain Osman, who allegedly tried to blow up a Tube train at Shepherds Bush.

Explore UK News

• Crime News

• Education News

• Health News

• Scotland News

Times Recommends

• The bright side of this winter rain
• Story behind the picture: Snow Leopard, 2008
• Times Charity Appeal: The Place2Be