Japan, the third-largest buyer of U.S. beef, has halted imports of the meat from a Wisconsin plant owned by JBS SA after finding noncertified beef products in a shipment, the U.S. Department of Agriculture said.

The cargo from JBS Green Bay Inc. in Green Bay, Wis., had beef tongues from cows that may not have been under 20 months of age, USDA Food Safety and Inspection Service spokeswoman Amanda Eamich said. U.S. exporters shipping to Japan must verify that products are from cattle under 20 months old and do not contain materials that might be linked to mad cow disease.

"They were unable to determine if certain portions of the shipment were under 20 months," Eamich said Friday in a telephone interview from Washington. "It was not a large shipment."

Japan, once the largest importer of U.S. beef, imposed restrictions on U.S. beef after a case of mad cow disease was discovered in Washington state in 2003.

"The shipment arrived after an apparent mispack," Eamich said. Her agency is investigating, she said.

"Japan asked that we look into it to determine what happened, and pending results of that, then we'll take our next steps," she said.