By Carey Gillam

(Reuters) - Monsanto Co. said on Wednesday that broad testing of commercial wheat seeds in Oregon and Washington state has found no sign of its long-shelved experimental biotech wheat, and company officials said it was possible the illegal wheat discovered growing in an Oregon field may have been the result of sabotage.

Monsanto officials said they have supplied DNA material and an event-specific test to the U.S. Department of Agriculture to help screen commercial supplies for any contamination of the company's genetically modified herbicide-tolerant wheat known as Roundup Ready. The European Union, Taiwan, Korea and Japan have also requested and received the test.

Monsanto said in 2004 it was stopping its biotech wheat research and said the last field trials ended in 2005. But the U.S. Department of Agriculture disclosed last week that the Roundup Ready trait was found in wheat growing on an Oregon wheat farm in April. The news has roiled international wheat markets because the wheat was never approved for commercial use.

Discovery of the genetically modified wheat prompted Japan and South Korea to cancel orders of U.S. wheat, and has supplied critics with more ammunition to challenge the safety of genetically modified crops.

The company said its investigation used its "CP4" event-specific test to examine 30,000 samples of 50 varieties, representing 60 percent of white wheat acres in Oregon and Washington. It said it found no presence of its Roundup Ready herbicide-tolerant trait, which Monsanto refers to as CP4.

"All varieties are clean of this event in wheat. This is a critical development," said Monsanto Chief Technology Officer Robb Fraley in a conference call with reporters on Wednesday.

Fraley said many testing methods used by others searching for the herbicide-tolerant trait were not accurate for testing wheat and its event-specific test was the only test that was valid.

He said the company did not know what type of testing was used to determine that Monsanto's biotech wheat trait was present in the Oregon field. The company does not even know the particular wheat variety that was impacted as it awaits information from the USDA, he said.

"This is the only reliable test," said Fraley. "The testing is complex. Sophisticated methods are required."

SABOTAGE?

Fraley said testing it has conducted over the last week has allowed the company to determine that the incident is "a random isolated occurrence more consistent with the accidental or purposeful mixing of a small amount of seed during the planting harvesting or during the fallow cycle in an individual field."

"What is going on? This is an important question," said Fraley, who emphasized he was not pointing a finger of blame at the farmer. He said sabotage was a possibility.

"We are not ruling anything out at this point. We know that the circumstances are highly unusual. We're going to continue to do the research until we get to the answer," said Fraley

The company has ruled out a number of scenarios to explain the presence of the GMO wheat, including pollen flow or the sudden natural growth of genetically modified wheat that the company said was last field tested in Oregon 12 years ago.

Monsanto said when it shut down its experimental Roundup Ready wheat program it had all of the research participants either destroy their supplies or ship them to a USDA seed storage facility in Colorado.

(Reporting By Carey Gillam; Editing by David Gregorio)