Meat meant for dog food – which contains hides, hair, feathers and bones – could have ended up in the human food chain, a report has warned.
A review into food supply said the industrial-scale fraud came to light in 2005, but was not followed up.
The many leads into crime networks were not pursued, the Elliott review said.
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It said the potential profit on about 1,000 tonnes of meat found at the cold store was in the region of £3m.
The report said the material found was most likely "part of an ongoing illegal business but for lack of the necessary investigative resource this was not pursued.
"The many leads that were opened into food crime networks at this time were not followed up."
The material included animal parts which have been passed as fit for human consumption but are not intended for consumption, either because they are parts of animals which are not usually eaten or for commercial reasons.
The case study was included in a report published by the Department for Environment, Food and Rural Affairs and written by Queen's University Belfast academic Prof Chris Elliott.
Irregularities were noticed when a shipping company informed Northern Ireland's food port inspectors of a suspicious container which had arrived from Asia.
This prompted a follow-up operation at a cold store and police obtained a warrant. The presence of an industrial shrink wrapping machine raised concern.
"It led on to the discovery of other packing and labelling equipment and materials including a copious supply of forged veterinary health marks purporting to originate from a variety of meat plants across the EU," the document said.
"It became evident that the primary business of the cold store was repacking and relabelling as fit for human consumption Category 3 animal byproducts meat."
An advertisement encouraged others in the industry to cut their losses by sending out-of-date stock to the illegal relabelling service.
"Evidence was found to demonstrate extensive criminal planning," the report added.
"At that time (or currently) there was no capacity for the major criminal investigation that the evidence and the criminal profits justified."
The meat was worth about £1 a kilo as pet food but if sold as mince for human consumption it would be worth £3.50 a kilo.
There were a large quantity of beef fillets and Prof Elliott said if that was combined with the mince the average price would have been at least £4 a kilo, making the criminal profit on the meat in the store worth between £2.5 million and £3 million.