The UK's National Crime Agency is investigating whether a significant cache of weapons found in Co Down could be linked to an organised crime gang operating in the Republic.
Eleven weapons, including handguns, machine guns, machine pistols and an assault rifle, and a quantity of ammunition were found in two waterproof holdall bags buried in a wooded area in Jerrettspass.
The firearms have been described as being in good condition and ready to use.
The search operation was conducted by specialist teams from the PSNI.
At this stage the NCA, described as the British equivalent of the FBI, does not believe the weapons belonged to a paramilitary organisation,
David Cunningham, the NCA's Branch Commander, described it as the most significant weapons find in Northern Ireland in a decade.
"This find is the result of our inquiries into organised crime across the UK. It's very significant to find weapons of this calibre and quality in one place," he said.
"We're satisfied at the moment, this is linked to organised crime. That said, because of the nature of the weapons, and, you know, this area of the country, we're keeping an open mind and we're hoping that the forensic examination will assist us.
"This is an organised crime investigation across the UK, this element of it relates to Northern Ireland. Once we can properly assess what we've recovered, that will inform us whether there are groups from the wider island of Ireland, or elsewhere, involved."
Additional reporting Press Association