A number of items have been found in a search of the Corcrain River in Portadown.
A police spokesperson said these included four suspected pipe bomb type devices and working parts of a firearm.
The pipe bomb type devices are not viable.
The operation in the Obins Street/People’s Park area has now ended.
Inspector Leslie Badger said, “The investigation is at an early stage and we are working to establish how old the items are and whether this find is linked to historic terrorist activity.”