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Monsanto Co’s experimental genetically engineered wheat, never approved for sale, has been found growing in a second U.S. state, and regulators said on Friday they could not explain how the plants escaped field trials that ended almost a decade ago.

About a year after discovery of the company’s unapproved wheat in a single Oregon field disrupted U.S. wheat export sales, the GMO wheat has also been found in Montana, the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service said on Friday.

APHIS launched an investigation into the Montana discovery on July 14, said Bernadette Juarez, director of investigative and enforcement services for APHIS.

The wheat was found growing at a research facility for Montana State University in Huntley, where field trials of Monsanto’s wheat were conducted between 2000 and 2003, she said in a news conference.

After conclusion of field trials, crop developers like Monsanto are obligated to inform regulators of any “volunteers,” or plants that grow on their own following a previous harvest, Juarez said.

Monsanto said in a statement that it and Montana state notified APHIS of the unintended GMO wheat and were “cooperating fully” with the government investigation.