DUBLIN (AP) — Ireland's prime minister vowed Tuesday to identify who has been putting horsemeat into Irish-produced burgers, a scandal that is casting doubts on the integrity of processed beef products across Europe.

Yet even as Prime Minister Enda Kenny said the problem had been linked to imported offcuts of Polish meat, experts said horse could have been added to burger-bound beef later in the supply chain, — and noted past examples of food-labeling fraud in Ireland's meat industry.

The Food Safety Authority of Ireland emphasized that the problem was a matter of honest labeling, not safety, and must involve fraud by a producer or supplier somewhere along the seven-nation journey by truck from Poland to Ireland. The reputational damage to Ireland threatens to erode international confidence in the country's top agricultural product, beef, a business worth €1.9 billion ($2.5 billion) a year to this country of 4.6 million.

"Clearly this is a matter that has to be sorted out ... a matter of reputation. Obviously we can't afford to have that," Kenny said as he entered a Cabinet meeting focused on efforts in Ireland, Britain and Poland to identify the source.

Asked whether the suspect meat product might have been misleadingly labeled as Polish by Irish fraudsters, Kenny said, "Clearly plants in Poland have been supplying material, but the evidence might be that other investigations need to take place as well."

Irish police have begun to investigate British and Irish meat traders involved in buying the Polish product and selling it on to Silvercrest and Rangeland, two Irish producers of frozen beef burgers in County Monaghan. Agriculture Minister Simon Coveney, who was testifying to lawmakers Tuesday, said Ireland's two other major producers of frozen beef burgers did not import any meat from Poland.

DNA testing on dozens of frozen burgers in Irish stores commissioned by the Food Safety Authority of Ireland uncovered the apparent fraud. The first results published Jan. 15 found 29 percent horsemeat content in one Silvercrest-produced burger for Tesco, the biggest supermarket chain in Britain and Ireland. Subsequent DNA tests by the British supermarket Co-op found another Silvercrest-made burger with 18 percent horsemeat.

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And on Monday night, Ireland's Agriculture Department said an original batch of the Polish-labeled product found in the deep-freeze storage at Rangeland was 75 percent horsemeat. Hours later across the border in the British territory of Northern Ireland, tests on similar product at a cold storage unit and earmarked for delivery to Silvercrest was found to be 80 percent horsemeat.

Both Silvercrest and Rangeland have suspended operations. Silvercrest already has lost its major supply contracts with Tesco and Burger King.

Alan Reilly, chief executive of the Food Safety Authority, said the latest findings demonstrated that Ireland was dealing with a certain case of fraud, not an accident.

"We're no longer talking about trace amounts of horse DNA in product. We're talking about horsemeat. Somebody, someplace, is drip-feeding horsemeat into the burger manufacturing industry. We don't know yet exactly where this is happening. All the documentary checks that we have on these shipments show that they have come from Poland," Reilly said.

He said the paperwork indicated Silvercrest imported some of the Polish meat directly, but other paperwork found that two meat traders in the United Kingdom and another in the Republic of Ireland were involved in other shipments.

Asked if he was confident the horsemeat came from Poland, Reilly demurred, noting that Polish veterinary and food safety authorities had yet to provide any official results from testing there.

"I'm confident that the products are labeled as Polish. All the documentation says these products have come from Poland," he said.

Susan O'Keeffe, an Irish senator whose work as an investigative journalist two decades ago blew the lid on corrupt practices among Irish beef exporters, said the paper trail could not be trusted to identify the horsemeat fraudsters. She noted that Irish producers two decades ago were caught mislabeling meat that was past its sell-by date and exporting it to Russia and other countries.

"People were employed to cut, scrape labels off frozen meat, and put their own stamps on it," O'Keeffe said. "You could do it with the meat itself, and you could do it with the box. You could forge labels. You could write your own labels. You could print your own labels. All of this did happen in our time."

O'Keeffe said the relied on a paper trail which could be forged. "It doesn't naturally follow that the meat came from Poland or that the meat was Polish," she said. "It might be — but it may not be."