The US Department of Agriculture has said that it has not yet determined how banned biotech StarLink corn ended up in a US cargo bound for Japan.

The cargo was detained by Japan’s Ministry of Health, Labour and Welfare on 27 December, after traces of StarLink corn were found.

The USDA has said that US grain inspectors had examined the vessel before it left for Japan and had not found any trace of StarLink corn.

“The barges that made up this shipment were tested under conditions that met the established protocol,” Dave Shipman, deputy administrator for the USDA’s Federal Grain Inspection Service was quoted as saying by Reuters. “All test results were negative.”

StarLink corn, which is only approved in the US for animal feed, caused controversy in 2000 after it was found to have entered the US human food chain. The corn is not even approved for use as animal feed in Japan, reported Reuters.