The experts have changed their story again. Now we're back to the notion of simple homemade devices. From the Guardian:

The simple but lethal device

Alok Jha, science correspondent
Saturday July 16, 2005

Police suspect that the explosive used in last week's attacks in London was a compound known to chemists as "mother of Satan".

It is simple to make and has been the explosive of choice for extremist groups in the Middle East for many years. It is often employed by bombers in the West Bank, and was used by the shoe bomber, Richard Reid, who had packed it into his shoe in his failed attempt to bring down an American Airlines flight in 2001.

Its chemical name, acetone peroxide, betrays the simplicity of its ingredients, which can be drawn from easily available household products such as nail varnish remover.

A polymer of acetone peroxide - triacetone triperoxide (TATP) - is one of the most sensitive explosives known, being highly sensitive to impact, temperature and friction. It can be made to explode in the presence of a flame or spark.

"It is very, very dangerous," said Hans Michels, an explosives expert at Imperial College.

"The skills of a reasonable graduate from a British university first degree who has done organic chemistry will perfectly well understand how to make it, and he could make it."