Graham Keegan said the Roughton Woods area was renowned locally as a Second World War ammunition dump and discoveries were regularly made.
However, he added the involvement of mustard gas suggested a potential link with the First World War.
Mr Keegan said: “You have to ask whether there was a facility in WW1 for storing chemical weapons.
“If there was, then there are no records and no-one knows anything about it - apart from the military.”
Mr Keegan explained mustard gas was widely used in WW1 - but not in WW2.
He backed calls by police for people to stay away from the area.