The Agriculture Dept. says it has no indications that genetically modified wheat found in the western state of Oregon last month has spread beyond the field in which it was found.
No genetically engineered wheat has been approved for U.S. farming, and the department is investigating how the engineered wheat got in the field.
Japan, Korea and Taiwan have suspended imports of western white wheat from the Pacific Northwest as the USDA investigates.
USDA spokesman Matt Paul said in a statement Friday that the department "has neither found nor been informed of anything that would indicate that this incident amounts to more than a single isolated incident in a single field on a single farm."
Agriculture Dept. officials have said the wheat is the same strain as a genetically modified wheat that was designed to be herbicide-resistant and was legally tested by seed giant Monsanto a decade ago but never approved.
Paul said that investigators have interviewed the farmer who harvested the wheat from his field, interviewed the supplier who sold the producer wheat seed, obtained samples of the seed and obtained samples of other wheat grown by the farmer. All of the samples have tested negative so far, Paul said, and there is no indication that the engineered wheat entered commerce.
USDA said the investigation is continuing and inspectors are conducting interviews with approximately 200 area growers. The department also has given trading partners a copy of the test developed by Monsanto so they can identify the engineered strain.
The modified wheat was discovered when field workers at an Eastern Oregon farm were clearing acres for the bare offseason when they came across a patch of wheat that didn't belong. The workers sprayed it but the wheat would not die, so the farmer sent a sample to Oregon State University to test.
A few weeks later, Oregon State wheat scientists discovered that the wheat was genetically modified. They contacted the USDA, which ran more tests and confirmed their discovery.
Most of the corn and soybeans grown in the United States are already modified, or genetically altered to include certain traits, often resistance to herbicides or pesticides. But the country's wheat crop is not, as many wheat farmers have shown reluctance to use genetically engineered seeds since their product is usually consumed directly. Much of the corn and soybean crop is used as feed.
The USDA has said the wheat would be safe to eat if consumed. But American consumers, like many consumers in Europe and Asia, have shown an increasing interest in avoiding genetically modified foods.
There has been little evidence to show that foods grown from engineered seeds are less safe than their conventional counterparts, but several state legislatures are considering bills that would require them to be labeled so consumers know what they are eating.
Representatives for Monsanto Co. said June 5 that they believe the emergence of the genetically modified strain was an isolated occurrence.