Police have arrested a 42-year-old man on suspicion of terrorism-related offences after two suspicious packages were sent in the post.
The man is being held under the Terrorism Act
Parcels containing caustic soda - which can cause blindness or burns - were sent from the Greater Manchester area to a councillor and a journalist.
Each parcel contained a letter and a miniature spirit bottle containing the potentially lethal substance.
Police have stressed there is no widespread risk to the public.
The man was arrested on Monday evening at his home in Tyldesley which is now being searched. He is being questioned by anti-terror officers on suspicion of the commission, preparation and instigation of acts of terrorism.
The recipients of the two parcels were based in Blackburn and Glasgow but no link has been made between them at this stage.
Police said the package addressed to the councillor was intercepted at the sorting office while the other one was delivered.
It is believed that if either person had drunk the contents of the miniature spirit bottle, they may have died within minutes without medical attention.
Caustic soda is widely used in industry for making paper, pulp, textiles, soap and detergent but contact with it in concentrated form can cause serious injury.