The Japanese government tried to explain Wednesday how beef containing trace levels of radioactive cesium ended up reaching stores or consumers in several prefectures, and promised to step up inspections to prevent another such incident.

News reports said cows raised at a farm near the damaged Fukushima Daiichi nuclear power plant likely had eaten rice straw contaminated with cesium.

The cows were shipped sometime between May and June to a Tokyo meatpacking plant, where some of the meat was found to have three to six times the allowable level of cesium. Although news reports earlier this week said none of the meat from the animals entered the market, later reports said some of it had been sold and possibly consumed.

Authorities insisted there was no need to worry about adverse health risks given the small portions of meat that would have been consumed.

According to the Mainichi Daily News, Charles Casto, deputy regional administrator at the U.S. Nuclear Regulatory Commission, expressed dissatisfaction with Tokyo’s response to the matter.  The Japanese government responded by saying it would be working to shore up its monitoring program.

Since the earthquake and tsunami damaged the nuclear plant four months ago, milk, vegetables, fish and tea leaves in Japan have been found with radiation contamination exceeding legal limits.