An expert on fertilisers at the University of London, Prof Andrea Sella, told the BBC it was possible that the highly explosive ammonium nitrate had also been stored on site.
At least two of them had been stuffed with some 1, 400kg (3, 086lbs) of ammonium nitrate not much less of the fertiliser explosive than was needed to murder 168 people in the 1995 Oklahoma City bombing.