TOKYO (Kyodo) -- The Japanese government said Wednesday it has suspended beef imports from a U.S. plant after two packages of chilled beef were found to contain spinal columns, a risk material for mad cow disease, at Tokyo port.

The two packages were among 16 tons of U.S. beef flanks and other parts in 810 packages which arrived at the port from Creekstone Farms Premium Beef LLC's plant in the state of Kansas, according to the Ministry of Health, Labor and Welfare and the Ministry of Agriculture, Forestry and Fisheries.

Spinal columns were found when the Animal Quarantine Service examined 28 of the 810 packages on Tuesday. No risk materials for the disease, formally known as bovine spongiform encephalopathy, were found in the remaining 808 packages.

The ministries asked the U.S. Department of Agriculture to investigate why the two packages were among those shipped to Japan.

Japan has limited U.S. beef imports to those from cattle aged 20 months or less since the first U.S. case of BSE discovered in December 2003.

Japan's import conditions also include the removal of brains, spinal columns and other specific risk materials that could cause the disease.

Since Japan resumed beef imports from the United States in July 2006, there have been 12 violations of the bilateral beef trade agreement committed by U.S. meatpacking plants, including the latest incident which is the second case involving a BSE risk material.

Japan has imported a total of 148,000 tons of beef from 41 U.S. plants since the resumption of imports, including 9,000 tons from Creekstone' Kansas plant.