Customs officers and gardaí in Co Sligo have seized an oil tanker and almost 17,000 litres of laundered diesel.
The diesel, which is valued at nearly €15,000, was found in the oil tanker and at a local filling station.
The driver of the tanker and the owner of the filling station have been released following questioning, a statement from the Revenue Commissioners said.
A file is being prepared for the Director of Public Prosecutions.
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Officers from Revenue's customs service, with gardaí from Carrick-on-Shannon, led the joint operation.