Ten people are still being questioned about what police have described as a conspiracy to purchase weapons for dissident republicans.
They were arrested following raids in counties Armagh and Fermanagh on Monday.
Two hundred officers searched eight premises and detectives also went to France to help with a related search.
Those arrested are being held at a centre in Antrim. One is also being questioned about the Omagh bombing.
Earlier on Monday, police said they believed they had disrupted a "potential major terrorist conspiracy" in the UK and Europe.
The searches were part of a joint investigation "between the PSNI Crime Operations Department and the British Security Service", they said.
Detective Chief Superintendent Norman Baxter said the investigation had been ongoing for a number of months and involved close liaison with members of "the Security Service and police services across Europe".
"The investigation has drawn on all branches of the Police Service of Northern Ireland's Crime Operations Department and officers from that department have travelled to Europe to progress the investigation.
"We believe that this operation has disrupted a potential major terrorist conspiracy," Mr Baxter said.