His reply to Sidney Alford's letter (3 May) implied it is just as easy to improvise a large explosive device from urea as from ammonium nitrate. This is not true, since for urea to become a useful explosive ingredient it must be nitrated using concentrated acids. While this may be possible in your garden shed for a few grams, I doubt whether any competent terrorist would attempt to nitrate the tonne or so required for a massive bomb, even assuming he could obtain and transport the thousands of litres of concentrated acids needed without getting caught.
In short, to use urea for large devices such as those recently exploded in London is impracticable. To make a bomb from the currently freely available (and easily stolen) ammonium nitrate you only have to add one other ingredient, which can be bought anywhere without suspicion, to make a bomb limited in size only by the vehicle you can get away with transporting it in.
The 'only way forward' as I see it is not only to 'be as alert and security-minded as possible', whatever Mr Higgs means by that, but to lock up our ammonium nitrate.
M. J. RIBBANDS
Tibenham, NorfolkReuse content