Andy Oppenheimer, an explosives expert with Jane's Information Group, said that one possibility was that the bombers had used some of the home-made explosive left over from 7 July but that the bombs failed to go off because ithad deteriorated.
He said: "It seems highly unlikely that you would get four failed detonators. What is possible is that the explosive has deteriorated after being made two or three weeks ago. That would explain why the second bombing attempt had to be so soon and why they all failed to go off."
In the aftermath of the 7 July bombings police discovered a large quantity of explosives in a bath at a housein Leeds. Although initially this was said to be high-grade military explosive, it was later acknowledged to be home-made, possibly acetone peroxide, which can be made from easily available chemicals. At least one of the four bombs left behind in Thursday's failed bombing attack is believed to have been a nail bomb. The bomb which failed to detonate on the No 26 bus in Hackney is thought to have been packed with nails and nuts and bolts.
The unexploded rucksack bombs are expected to provide a rich source of information for police and the security services.
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