MINAMISOMA, Japan—Within days of the disaster at the Fukushima Daiichi nuclear plant, Japanese food inspectors were spot-checking meat from the region's slaughtered cattle for radioactive contamination. Officials later fanned out to farms near the crippled plant to pass Geiger counters over the animals to determine whether they were safe to sell.

"We urge consumers to continue shopping as usual and retailers to do their business as usual," Agriculture Minister Michihiko Kano reassured the nation on March 31, more than two weeks after explosions at the plant first spewed radiation into the air.

That advice turned out to be misguided. On July 8, government officials testing meat from a Tokyo slaughterhouse said they detected levels of radioactive cesium at nearly five times Japan's limit. The contaminated beef was traced to a farm here about 16 miles north of the damaged plant—from an animal whose hide had been checked by inspectors.

The revelation has raised all kinds of questions about how much contaminated beef had already been consumed, kicking off a food scare that continues to grow as more tainted meat is discovered.

A Wall Street Journal examination shows serious flaws in Japan's approach to safeguarding food in the event of a nuclear accident. Four months after the disaster, the government still is struggling to contain the contamination and to come up with an effective system for policing its food supply. Some foods, such as juices and honey, hit store shelves without any government screening. Many other foods are spot tested, but only minimally.