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Correction: This story was updated to reflect that the Southern Agricultural Research Center is affiliated with Montana State University, not the University of Montana.

The discovery of rogue genetically modified wheat plants growing in an Oregon field last year was officially deemed an unsolved mystery by the U.S. Agriculture Department on Friday, even as it revealed that a similar incident in Montana is under investigation.

The new discovery involves unapproved genetically engineered (GE) wheat plants found growing at a University of Montana research center, adjacent to fields where the herbicide-resistant plants were field tested from 2000 to 2003.

While the new discovery raises questions about the regulation of such authorized tests, it is unlikely to generate the same level of alarm as the Oregon find on May 3, 2013, which involved a commercial wheat field. That prompted Japan, South Korea and Taiwan to temporarily suspend purchases of western white wheat grown throughout the Pacific Northwest.

The USDA says that while GE wheat has not been approved for sale or commercial production in the U.S., it does not pose a food safety threat. But foreign markets have rejected GE wheat, even as they have embraced other genetically engineered crops like corn, perhaps because it is so ubiquitous in processed food products.

The USDA’s Animal and Plant Health Inspection Service (APHIS) said in a news release that the two U.s. appearances of the GE wheat in the wild do not appear to be linked, as genetic testing showed the plants were from different stocks.

Bryan Cranston of Moro, Oregon, holds his daughter Claire, 2, in a freshly harvested wheat field on July 19, 2013. Carl Kiilsgaard / for NBC News

In the Oregon case, a small number of wheat plants genetically modified to resist the Monsanto herbicide Roundup were found growing on a single field. The service said its “thorough and scientifically detailed “ investigation had been closed without determining how they got there.

“After exhausting all leads, APHIS was unable to determine exactly how the GE wheat came to grow in the farmer’s field,” it said.

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The release said that the presence of the “volunteer” plants was apparently an isolated incident and that more than 100 laboratory tests of seeds and wheat harvested from the unidentified farmer’s field found no evidence that the genetically engineered wheat had made its way into commercial products.

The agency also indicated that the wheat plants apparently were not descendants of genetically engineered wheat plants field tested in Oregon as recently as 2001, saying they bore “genetic characteristics … (that) are representative of a wheat breeding program.”

Ed Curlett, a spokesman for APHIS, explained that wheat breeders generally cross varieties to accentuate certain beneficial traits, such as resistance to a certain disease or bug, specific to the region. During such experimentation, he said, the plants initially exhibit “a lot of genetic diversity,” which is reduced as the researchers focus in on specific desirable traits. The Oregon plants appeared to be in the early stages of that process, he said.

The genetically engineered wheat plants found on July 14 at the Montana State University’s Southern Agricultural Research Center in Huntley were scattered over 1 or 2 acres adjacent to fields where Roundup-resistant wheat was field tested from 2000 to 2003 under a contract with Monsanto, according to university spokesman Tracy Ellig.

He said that scientists at the center carefully followed the USDA’s research compliance requirements, including monitoring after the field test concluded.

“This entire time we’ve always been in compliance with what USDA dictates with field testing,” he said. “We’re cooperating fully and whatever recommendations they make we will certainly follow them going forward.”

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Charla Marie Lord, a spokeswoman for Monsanto, said that the APHIS investigation of the Oregon incident “affirms that no genetically modified wheat is in commerce and that the commercial seed and grain supply does not contain genetically modified wheat.”

As for the Montana discovery, she likewise pointed to APHIS’ conclusion that there is no indication the wheat was used in any commercial product and said “Monsanto is fully cooperating with that investigation.”

In a statement provided to NBC News, Philip Miller, head of Monsanto’s global regulatory affairs division, said the company also continues to refine its field test practices.

“While we believe our compliance program is best in class, we continuously review our processes and procedures to improve them, including site selection, field trial isolation and verification and auditing of field trial locations,” he said.

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But critics say that field tests of GE crops have frequently led to contamination of natural crops. And in the case of wheat, such accidental escapes would damage farmers’ livelihoods and cripple the U.S. export market, they say.

“Just as the USDA closes one fruitless investigation, it tries to bury the story of yet another contamination,” Andrew Kimbrell, executive director of the Center for Food Safety, a nonprofit public interest and environmental advocacy organization, said Friday in a statement. “USDA cannot keep treating these as isolated incidents; contamination is the inevitable outcome of GE crop technology. It’s time for Congress to take definitive action.”