Monsanto's unapproved, experimental genetically engineered wheat, which is feared to have potentially contaminated U.S. wheat supplies after it was found growing in an Oregon field this spring, was kept in a U.S. government storage facility until at least late 2011, according to documents obtained by Reuters.

The revelation that the seed for the controversial genetically engineered wheat was kept viable in a Colorado storage facility as recently as a year and a half ago comes as the U.S. government is investigating how the strain of experimental wheat wound up growing in an Oregon field this spring.

The probe by the U.S. Department of Agriculture includes an examination of the handling of the GMO wheat seed that Monsanto directed be sent to the government-controlled National Center for Genetic Resources Preservation in Fort Collins, Colorado, beginning in late 2004, according to Peter Bretting, who oversees the center for the USDA's Agricultural Research Service.

David Dierig, research leader at the National Center for Genetic Resources Preservation, also said the matter was "under active investigation."

The National Center uses high-tech methods to extend the viability of seeds for decades, much longer than their viability in conventional storage. The facility took in at least 43 physical containers of Monsanto's so-called "Roundup Ready" wheat in late 2004 and early 2005, the documents show. The material represented more than 1,000 different unique varieties or lines, according to the documents that Monsanto provided in a heavily redacted format. The documents were made up of correspondence between Monsanto and the Colorado facility.

Monsanto was shutting down its work with Roundup Ready wheat, altered to tolerate treatments of Roundup herbicide, when it set up a contract dated Nov. 2, 2004, for the resources preservation center to store its wheat seed. Monsanto said the seed was confirmed incinerated on Jan. 5, 2012.

"At our direction, the seed was destroyed ... as it was old material and we had no plans for its future use," said Monsanto spokesman Thomas Helscher, who provided Reuters with the supporting documents. Monsanto also archived some of the wheat at its facilities in St. Louis, Missouri.

When asked if USDA had accounted for all the supplies sent to the Colorado facility, USDA spokesman Ed Curlett said the government probe is seeking an answer to that question. A USDA spokesman on Friday said the government does believe that all the seed it received was incinerated, and that it cannot account for seed that might have been sent elsewhere.

The Roundup Ready wheat was never approved for commercial use and was supposed to be tightly controlled. Monsanto has said it suspects someone covertly obtained its wheat seed and planted it in the Oregon field to sabotage Monsanto's work with biotech crops. The government and Monsanto have said there is no indication the GMO wheat made it into commercial supplies, but the finding has hit Monsanto and the wheat industry hard.

Monsanto has been named in several lawsuits and over the last month, exports of U.S. western white wheat have been curtailed as foreign buyers shun the U.S. supplies and demand assurances that none of the biotech wheat has contaminated the marketplace.

Wheat growers want the mystery solved. "Determining how it happened would certainly make it easier for us to make sure ... that it doesn't happen again, regardless of whether it was sabotage or some accident," said Blake Rowe, chief executive of the Oregon Wheat Commission. "Our customers would like to know how it happened."