Monsanto officials said Friday that continued extensive testing of U.S. soft white wheat supplies shows that the presence of the company's unapproved, experimental genetically altered wheat in an Oregon wheat field is highly suspicious and was an isolated incident that could not have happened through normal farming practices.

Company officials said more investigation is needed to determine how the genetically engineered wheat, which Monsanto said it stopped field testing in 2005, was growing in April in the Oregon farm field.

"What happened in this field... is suspicious," said Monsanto Chief Technology Officer Robb Fraley in a conference call with reporters.

Fraley said the evidence indicates someone intentionally introduced the biotech wheat seed into the Oregon field.

Monsanto said Friday that testing it had conducted, in addition to testing conducted by Washington State University, found no sign of contaminated wheat outside that one field. The sampling represents over 97 percent of Oregon wheat acres, Monsanto said.

"The grain is clean," Fraley said. "This situation is extremely isolated, with all the testing data again concluding that this is isolated to a single field in Oregon."

The U.S. Department of Agriculture announced May 29 that it was investigating the discovery of genetically engineered wheat plants developed by Monsanto to tolerate dousings of Roundup herbicide. The "Roundup Ready" wheat was found on an Oregon farm in April, several years after Monsanto stopped field testing the wheat, which was never approved for commercial use.

Over the last month, exports of U.S. western white wheat have been curtailed as foreign buyers shun the U.S. supplies and demand assurances that none of the biotech wheat has contaminated the marketplace.

One area of inquiry has been what happened to GMO wheat seed sent to a government seed storage facility in Colorado when the field trials ended in 2005. Monsanto officials have said that some of the experimental wheat was shipped to the Colorado facility, called the National Center for Genetic Resources Preservation.

The center uses high-tech methods to keep seeds viable for decades, much longer than they typically would remain viable. Officials with the center said this week that they were not certain if they had received the GMO wheat seed and if so what might have happened to it.

USDA said it was investigating that issue, but Monsanto said Friday that all the seed sent to the Colorado storage site was destroyed.

"We have documentation of what seed was sent to the Colorado facility and documentation of its subsequent destruction," spokesman Thomas Helscher said. "At our direction, the seed was destroyed (incinerated) as it was old material and we had no plans for its future use."

Millers, grain handlers, exporters, wheat growers and others have complained that USDA/APHIS officials are not disclosing enough information about their findings, and until they do, the market for western white wheat will remain limited.

On Friday, USDA spokesman Ed Curlett said the investigation was proceeding and that so far the testing has focused on the three varieties of soft white wheat seed that the farmer in Oregon who found the Roundup Ready wheat had planted on his farm since 2009.

Investigators obtained samples of the same varieties of wheat seed sold to the farmer and other growers, and obtained samples of the farmer's wheat harvests, Curlett said. Investigators have also identified over 250 farmers who purchased and planted the same seed varieties and conducted nearly 230 in-person interviews with these farmers who all said they had not found any glyphosate resistant wheat volunteers on their farms.

The government has tested eight samples of seed and four grain samples and none of the more than 100 tests conducted have turned up positive for the experimental genetically engineered trait, according to Curlett.

Curlett said the government tested nine "pools" from each of the 12 samples for detection of as small a contamination level as 0.003 percent, or roughly one in about 30,000 kernels.