Fuel laundering plant found near border at Monaghan
More than 37,000 litres of laundered diesel has been discovered close to the border in the Irish Republic.
Police and customs officials have described the discovery in Castleblaney, County Monaghan, as one of the biggest in recent times.
Customs officers, backed up by armed police, raided the premises at 0600 BST on Thursday.
A man in his 40s was subsequently arrested and is being questioned at Castleblaney Garda Station.
Security sources told the Irish state broadcaster RTE that dissident republican involvement in the operation could not be ruled out.
It is believed the plant was capable of producing 20m litres of fuel a year and could have cost the Irish exchequer 11m euros (£9.7m) in loss of taxes.
More than 37,000 litres of laundered diesel were discovered during the raid as well as tankers, vans and equipment the criminal gang used to remove the dye from agricultural diesel.
It appears the toxic waste produced by those involved was pumped out into a nearby field and from there into local streams and domestic water supplies.