American wheat farmers and a food safety advocacy group filed a lawsuit Thursday against biotech seed developer Monsanto Co, accusing the company of failing to protect the U.S. wheat market from contamination by its unauthorized wheat.

The petition, filed in the U.S. District Court for the Eastern District of Washington, seeks class-action status to represent other farmers it says were harmed by lower wheat prices as some foreign buyers have shied away from U.S. wheat.

It names Clarmar Farms Inc., farmer Tom Stahl, and the Center for Food Safety as plaintiffs.

The suit follows a similar action filed Monday by a Kansas wheat farmer, alleging that he and other growers have been hurt financially by the discovery of an unapproved biotech wheat that Monsanto said it stopped testing and shelved nine years ago.

Two other farmers lodged a similar lawsuit in federal court in the western district of Washington state.

The lawsuits come after the U.S. Department of Agriculture announced May 29 that a wheat farmer in Oregon had discovered Monsanto's experimental wheat growing on his farm.

"It risks one of the U.S.'s most important export markets unnecessarily," George Kimbrell, senior attorney with the Center for Food Safety.

Buyers in Asia and Europe shunned U.S. wheat purchases after the discovery of the rogue wheat in Oregon. South Korea and Japan have suspended some U.S. wheat purchases, while the European Union said it would step up testing.

The wheat was developed by Monsanto to withstand treatments of its Roundup weed killer, but the firm never commercialized the product because of widespread industry opposition. International buyers threatened to boycott U.S. wheat if the biotech wheat was introduced to the marketplace. Monsanto said in 2004 that it would end efforts to commercialize the "Roundup Ready" wheat.

This week, Monsanto said that in ending its field testing, the company instructed test participants to destroy the GMO wheat or ship it to the U.S. Department of Agriculture's seed storage facility in Colorado. Company officials have said they have no idea how their biotech wheat could be growing in Oregon.

Monsanto lawyer Kyle McCain called the lawsuits premature.

"The facts to date show the report of glyphosate-tolerant wheat is limited to one field in Oregon, and no such wheat has entered the stream of commerce," McCain said in a statement.

In the suit filed Thursday in Spokane, Washington, plaintiffs say Monsanto's failure to contain the wheat amounts to "wrongful conduct" that has potentially contaminated "the entire wheat farming and production chain," and places many wheat farmers at risk for continued harm through cross-pollination and contamination of their crops.

The petition says the claims are brought on behalf of all U.S. soft white wheat growers and harvesters from May 29.

The field testing Monsanto had been doing in many U.S. states was supposed to keep the biotech wheat from contaminating conventional wheat supplies.

The lawsuit does not seek specific monetary damages but asks for "compensatory damages" as well as punitive damages and ask that Monsanto be required to decontaminate equipment, storage and transportation facilities.

Monsanto says it followed a "government-directed, rigorous, well-documented and audited" program in its wheat field trials.

But Kimbrell said regulators do not provide proper oversight of experimental biotech crops.

"The lawsuit underscores the continuing risk to consumers and farmers from unregulated genetically engineered crops," Kimbrell said. "This is not the first time this has happened. We have a broken regulatory system."