An analysis of the failed attacks of June 28 in London’s West End -- in which cell-phone detonators failed to set off two car bombs each containing three large propane gas cylinders, dozens of gallons of gasoline and bags of nails -- is included in an August 2007 terrorism risk briefing produced for the insurance industry.

Such devices, which rely on creating an explosive mixture of fuel, air and gas, “are notoriously difficult to get right,” says the briefing, to be released to clients later this week by insurance consultants Risk Management Solutions.

“The difficulty of creating a large aerosol of a consistent explosive fuel-air mixture either in the open air or within contained spaces has thwarted even military attempts to make (such) devices,” the briefing notes.

Now that sales of ammonium nitrate fertilizer are increasingly monitored, the briefing says, and given the difficulties in making -- and relatively small explosive yields of -- peroxide-based bombs, the effort to use fuel-air techniques “was an inventive way to attempt a much larger explosion using unmonitored materials.”

But such devices are “much more technically challenging” than either fertilizer or peroxide-based explosives. “If terrorists are trying such techniques it reflects ambition over practical experience,” concludes the analysis.

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Shaun Waterman, UPI Homeland and National Security Editor