WASHINGTON — The Animal and Plant Health Inspection Service (APHIS) of the U.S. Department of Agriculture has concluded that the presence of bioengineered wheat found growing in a single field on a single farm in Oregon in May 2013 “appears to be an isolated incident.” APHIS said it closed the year-long investigation “after exhausting all leads.”

The bioengineered wheat found on the Oregon farm was developed by Monsanto to be resistant to the herbicide glyphosate, also known as Roundup.

APHIS began its investigation on May 3, 2013, when an Oregon State University scientist notified the U.S.D.A. that plant samples had tested positive for a protein that made them resistant to glyphosate, the department said. Over the next 10 months, APHIS conducted 291 interviews with wheat growers, grain elevator operators, crop consultants, and wheat researchers, and collected and reviewed thousands of pages of evidence. Additionally, APHIS collected more than 100 samples from businesses that sold and purchased the same certified seed planted in the field in Oregon, as well as from businesses that purchased the harvested grain the grower.

“The investigation indicates that this appears to be an isolated occurrence and that there is no evidence of any G.E. wheat in commerce,” APHIS said. “Information collected during the investigation was instrumental in providing critical information to trading partners to keep foreign wheat markets open. After exhausting all leads, APHIS was unable to determine exactly how the G.E. wheat came to grow in the farmer’s field. The investigation also found that the G.E. wheat is not a commercial variety of wheat. Instead, the genetic characteristics of the G.E. wheat volunteers are representative of a wheat breeding program.”

As APHIS closed the Oregon case it announced the opening of another investigation, this time in Montana. The agency said it was notified on July 14 of the discovery of bioengineered wheat at the Montana State University’s Southern Agricultural Research Center in Huntley, Mont., where Monsanto and researchers grew bioengineered wheat as part of field trials between 2000 and 2003. The field tests were conducted under APHIS’ regulatory approval.

According to APHIS, testing samples by a U.S.D.A. laboratory confirmed that the wheat is genetically engineered to resist Roundup, and further genetic testing showed that the wheat collected from the field in Montana was not the source of the wheat found growing in the single field in Oregon.

“Among other things, APHIS’ ongoing investigation is focusing on why G.E. wheat was found growing at the research facility location,” APHIS said. “G.E. wheat from the facility has not been allowed to enter commercial channels this year, and G.E. wheat grown as part of authorized field trials at this research facility between 2000 and 2003 was likewise not allowed to enter commercial channels. None of the wheat is sold as seed. APHIS will provide more information when it concludes its investigation.”

As it continues its investigation in Montana, APHIS said it is taking several additional steps to ensure that unintended bioengineered wheat is not growing in other locations in the United States where field trials are taking place or recently have occurred.

“APHIS will inspect field trials planted in 2014 and follow-up with post-harvest inspections to ensure those conducting the field trials adhere to APHIS’ requirements to monitor for, and remove, volunteer plants (plants that grow in a field following a previous harvest),” the agency said, adding it also will conduct some post-harvest volunteer monitoring inspections of bioengineered wheat field trials that were planted in 2012 and 2013.

“Beyond this, APHIS is assessing other measures — such as the requirements it puts in place for field tests involving G.E. wheat, as well as the frequency of its inspections of field test sites — to minimize the potential for any further incidents involving G.E. wheat,” the agency said.