In a separate development, tests commissioned by The Asahi Shimbun revealed that the supposedly pure beef products contained not only pork, but chicken as well.

Investigators searched the head offices of Meat Hope Co., in Tomakomai, and Hokkaido Katokichi Co., in Akabira, for evidence of anti-competitive behavior, sources said.

They said police suspected the companies of engaging in systematic fraud and would closely examine evidence with a view to filing charges that could carry heavier penalties.

The companies have already admitted falsely labeling their products. Meat Hope President Minoru Tanaka has acknowledged the practice started seven or eight years ago.

Hokkaido Katokichi bought meat products from Meat Hope and used them to manufacture frozen foods that were sold throughout Japan.

Tanaka said last week the company had used pork as a substitute for beef in company products labeled "100-percent beef." He said nothing about chicken.