An agricultural dragnet has found no cases of unapproved genetically engineered wheat outside of a 123-acre field in Oregon after two months of an ongoing investigation, the U.S. Agriculture Department said on Wednesday.

The discovery of the wheat, not approved for cultivation anywhere in the world, shook the global market when it was announced on May 29. Some U.S. customers in Asia and Europe halted purchases or said they would test incoming wheat shipments for biotech grain.

With the U.S. wheat harvest in its early stages, the Washington State Grain Commission expressed "utmost urgency" for the USDA to spell out how it will find and isolate any biotech wheat among the tens of millions of bushels grown in the Northwest and "ensure our markets are not compromised."

Some $9 billion worth of U.S. wheat, roughly 45 percent of the crop, is exported annually. Japan excluded U.S. white wheat from the Pacific Northwest from a tender this week, but said soft red winter wheat, typically grown in the U.S. Southeast, was acceptable as a substitute.

Investigators say the unapproved wheat was developed years ago by biotech leader Monsanto Co to resist applications of a widely used weed killer. Monsanto shelved the project and ended field tests in 2005 in the face of worldwide opposition to genetically altered wheat.

"The investigation continues," a USDA spokesman told Reuters in an e-mail. "We can confirm that testing associated with the investigation has so far been negative and that we have no information that GE (genetically engineered) wheat is in commerce.

"The focus of the ongoing investigation is the area surrounding the 123-acre field where the detection of the GE wheat volunteers has been confirmed."

Other fields on the Oregon farm were tested. The USDA has also said it was interviewing surrounding landowners.

Bob Zemetra, an Oregon State University wheat breeder and genetics specialist, said the university used different samples than the USDA when testing for the contamination. Both series of tests found positives.

"It was more than one plant. There were contaminants is that field," Zemetra said.

The USDA went public with the discovery after four weeks of on-the-ground sleuthing and sophisticated testing to confirm the wheat was a Monsanto strain. Unwanted seedlings sprouted in the spring in a wheat field, in northeastern Oregon, which was being held fallow this year.

When the plants survived a weed killer, the farmer sent samples to Oregon State University. In early May, the university alerted the USDA, which dispatched investigators within days.

No "rapid test" kits are available that are validated for biotech wheat. The USDA said its grain inspection agency "is working toward making available appropriate and validated testing techniques to address market needs that may develop."

Monsanto has shared with overseas regulators the test needed to identify its herbicide-resistant spring wheat.

Robb Fraley, Monsanto's chief technology officer, said last week that the company was working "night and day" to build up the testing capabilities. He said the needed testing methods are complex and sophisticated. Company officials said they provided the USDA with positive-controlled DNA material and the needed testing protocols.