From the Times:
http://www.timesonline.co.uk/article...690391,00.html

July 12, 2005

Terrorist gang 'used military explosives'

By Michael Evans, Sean O’Neill and Philip Webster
A SINGLE bombmaker using high-grade military explosives is believed to
be responsible for building the four devices that killed more than 50
people last week, The Times can reveal.

Similar components from the explosive devices have been found at all
four murder sites, leading detectives to believe that each of the 10lb
rucksack bombs was the work of one man. They also believe that the
materials used were not home made but sophisticated military explosives,
possibly smuggled into Britain from the Balkans.

“The nature of the explosives appears to be military, which is very
worrying,” said Superintendent Christophe Chaboud, the chief of the
French anti-terrorist police, who was in London to help Scotland Yard.

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Of course, a single bombmaker doesn't necessarily mean all the bombs
were placed by one person. But it increases the odds greatly.

Ed
anthraxinvestigation.com