This from the Financial Times, one month after the July 7th attacks:
Forensic experts seek clues to terror mastermind
By Bob Sherwood
Sunday, August 07, 2005
Posted: 01:41 PM EDT (18:41 London)
The precise make-up of the explosives, and the way they were triggered, are the crucial clues that investigators have been seeking to determine since the July 7 attacks.
While the manhunt for the July 21 would-be bombers was carried out with highly visible police raids, forensics experts working on the July 7 bombs have been focusing on the painstaking job of examining minute blast fragments.
There is plenty of evidence to work on. As well as the forensic items from the blast scenes, police seized material connected to the making of explosives during raids in Leeds days after the July 7 attacks. Bomb components were also recovered from a car left at Luton railway station by the bombers.
Scotland Yard said that so far there were only "surface similarities" between the two attacks, with no proof of a common bomb maker or guiding mastermind.
The home-made explosives were peroxide based, and the NYPD said the bombs used the explosive hexamethylene triperoxide diamine, known as HMTD, apparently made from the likes of hydrogen peroxide hair dye and citric acid food preservative.
That came as a surprise to explosives experts who had believed that the bombers had used the easier-to-make and less volatile triacetone triperoxide. Scotland Yard has confirmed only that the explosives were home made, such as TATP or "variations upon" it.
Professor Hans Michels, an explosives expert at Imperial College London, said: "I would be extremely surprised because TATP is easier to make and not as unstable as HMTD. If the bombs were made from this material, you would be really risking your lives even travelling with it.[/b]"
He said making a large quantity of HMTD in a house in a Leeds suburb would risk "blowing the whole street apart". However, it was possible a small quantity of HMTD could have been used, perhaps enclosed in a bottle, as an initiator, with another explosive packed around the outside to provide most of the force.
The use of HMTD would indicate the involvement of a highly trained, professional chemist."Whereas preparation of large amounts of medium grade TATP might conceivably be left to trained amateurs, any evidence of the use of HMTD would certainly point towards involvement of a specialist explosives chemist in preparation of these bombs," Prof Michels said.
John Wyatt, a counterterrorism explosives expert, said that, in spite of the proliferation of bomb-making instructions on the internet, few people are capable of putting together such volatile explosives. "The [July 7 and July 21 bombs] might not have been made by the same person, but I think there are probably only one or two people [in the country] capable of doing it."
Investigators say they are close to identifying the precise mix used in the first attacks. They will be trying to determine whether the July 7 explosives could have come from the same "recipe", or even the same batch, as the July 21 bombs that failed to go off.
Determining whether the explosives were made from the same instructions could be difficult. "Even if they were being made from the same recipe, different people would inevitably make it slightly differently," said Mr Wyatt.
The same batch of explosives for both attacks appears unlikely, as TATP becomes more, rather than less, volatile as it ages. Yet in the attacks two weeks later the bombs did not detonate. It would also be difficult to store HMTD for a long period, even in the kind of commercial refrigerator believed to have been used at the Leeds house.
A potentially more useful line of inquiry could be the "switching systems", the triggers, wiring and detonators, Mr Wyatt said.
"I think the switching systems are going to be extremely interesting to give the police an idea of what other cells might do."
The NYPD also said they had been told the bombs were triggered by mobile phone alarms set for 8.50am, rather than by the bombers detonating them manually. That is possible as the current from a phone alarm could easily be amplified and used to set off the blast cap. In addition, the devices would work underground as the phones would not be dependent on a signal.
If that proves to be true, it could suggest the men, who were carrying identification, had bought return tickets and left a car with explosive components at Luton station, did not intend to be, or did not know they were being used as, suicide bombers. Timers are used for planted bombs, whereas suicide bombers can trigger their devices manually when in the target position.
Source: Financial Times