The unapproved genetically modified wheat that was discovered sprouting in Oregon appears to have been "a single isolated incident," U.S. agricultural officials said on Friday in their most detailed description yet of their ongoing investigation.

The U.S. Department of Agriculture said in a statement it had found no genetically modified seeds in any field it investigated beyond the original site, and "all information collected so far shows no indication" any biotech wheat was in the food chain.

Investigators are interviewing "approximately 200 area growers," said USDA. It said investigators also interviewed the seed company that provided the seed.

USDA said it gave skittish trading partners a test method on Thursday to identify the wheat, developed years ago by Monsanto, which is not approved for cultivation anywhere in the world.

Importers of U.S. wheat have been clamoring for a test and U.S. farmers have pressed for an USDA explanation of how it was proceeding and assurance their crops are free of suspicion. The new U.S. winter wheat crop is in the early stages of harvest.

"As of today, USDA 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," said the statement.

The United States exports about $9 billion in wheat per year, and two countries canceled orders after the genetically modified wheat was discovered in Oregon. The investigation started after the discovery of "a small number of volunteer wheat plants" on a field in northeast Oregon that was being held fallow this year.

The unwanted seedlings survived spraying with the widely used weedkiller glyphosate, often sold under the Monsanto brand Roundup.

All seed and grain samples collected by investigators, aside from the initial, 123-acre field, have tested negative for the genetic modification by Monsanto that gives spring wheat a resistance to glyphosate, USDA said.

It tested samples of wheat sold to the farm where the biotech wheat was discovered and to other farmers as well wheat harvested from the farm, including the 2012 crop.

The Capital Press newspaper in Oregon reported on Wednesday that USDA took samples from a seed company in Walla Walla, Washington, and from some of the company's customers.

USDA has repeatedly said there is no public health risk from the wheat strain. Monsanto stopped work on glyphosate-tolerant wheat in 2005 in the face of worldwide opposition to biotech wheat. The last field trial in Oregon was in 2001.

Customers such as Japan, South Korea and Taiwan have shunned U.S. white wheat, the dominant variety in the Pacific Northwest, since the announcement May 29 of the discovery. The European Union said it would test incoming shipments.

To help trading partners make "science-based trade decisions," USDA said it provided a validated DNA-based method to identify the Monsanto strain at a rate of one grain out of 200.

Also on Friday, the EU Commission's scientific body published a validated test for the Monsanto strain.