A suspected fuel laundering plant in Tassagh went up in flames following an explosion on Thursday morning.
Fire crews rushed to the scene of the blaze at a farm building on the Armagh Road, close to Armaghbreague, shortly after 10am.
They were alerted by police crews who had observed smoke coming from outbuildings.
Soon after HMRC officers swooped to assist at the scene, following the discovery of a suspected laundering plant.
A police spokesperson told Armagh I that HMRC were now leading the investigation.
Chief Inspector Bernard O’Connor said: “Officers attended to find a barn engulfed in flames. They were joined by colleagues from Northern Ireland Fire and Rescue Service, who extinguished the blaze.
“No one was present at the time and there were no reports of injuries. Our enquiries are continuing.”
Six fire crews attended the incident – two from Armagh, and the rest from Keady, Newry, Warrenpoint and Cookstown, and such was the intensity of the blaze, it took over five hours to fully extinguish.
A NI Fire and Rescue Service spokesperson confirmed the call came in at 10.10am.
“We were called to a farm building on fire and firefighters wearing breathing apparatus used two foam jets and one hose reel jet to extinguish the fire.
“The cause of the fire is believed to have been accidental ignition and the incident was dealt with by 3.20pm.”
A HMRC spokesperson told Armagh I: “We can confirm that we had officers working in the Newtownhamilton area of Co Armagh yesterday supporting partner agencies, but are unable to release further information at this stage.”