Federal agriculture investigators took samples from a Walla Walla, Wash., seed distributor as they continue to chase down how genetically-modified wheat popped up in an eastern Oregon field, the Capital Press newspaper reported.

The Animal Plant Health Inspection Service has not released details about the investigation, but two sources told The Oregonian that the farmer who reported the rogue wheat plants bought his seed from Northwest Grain Growers, in Walla Walla.

An APHIS spokesman confirmed samples were taken at the company. So far, all tests have come up negative, he said. He provided no other details.

Company General Manager Chris Peha said APHIS investigators visited his facility but provided little information.

“They haven’t informed us of anything,” Peha told The Oregonian. “They didn’t share any information with me.”

Asked if the farmer bought seed from his company, Peha said, “I don’t know that.”

In an interview with the Capital Press, Peha said, “I think they are trying to determine if the GMO wheat was in the seed, but I don’t know for sure. “They will not answer any of our questions.”

The Capital Press, which closely covers Pacific Northwest agriculture, also reported that APHIS investigators took samples from a Heppner wheat field. The APHIS spokesman would not comment on the report.

The investigation began when the farmer clearing an unplanted field of weeds sprayed “volunteer” wheat plants with a glyphosate-based herbicide. Oregon State University and APHIS scientists determined the plants carried a “Roundup Ready” modified gene that makes them resist the herbicide developed by Monsanto Co.

Monsanto field tested the variety in 16 states, including Oregon, from 1998 to 2005, but withdrew its application to have the wheat approved because farmers said export markets would not buy genetically-modified wheat.

The last testing in Oregon reportedly was in 2001, and the field in question was not a test site, deepening the mystery of how the GM plants appeared there.