YAKIMA, Wash. - A ground beef recall that has expanded to WinCo Foods stores in six Western states was prompted by a law firm's investigation of contaminated beef products.

Saying the meat could be contaminated with E. coli, California officials issued the recall Sunday for WinCo Foods fresh ground beef that was packaged in Styrofoam trays at the stores and marked with sale dates from March 28 to April 9. The warning covers about 70 stores in California, Idaho, Nevada, Oregon, Utah and Washington.

The announcement expanded a voluntary recall last week at one store in Modesto, Calif. California officials say an additional sample from that store tested positive for E. coli Friday, prompting the expanded recall. No illnesses have been reported.

The supermarket chain learned about the bacterial contamination from an independent lab that was conducting a nationwide survey of ground beef for Marler Clark, a Seattle law firm specializing in food-borne illness cases.

The ground beef likely came from one of two national beef companies that supply many grocery stores, said Michael Read, WinCo Foods vice president of public and legal affairs.

WinCo has no reason to believe any ground beef that was sold was contaminated, he said. Read had no estimate of how many pounds of ground beef could be affected, but he noted that much of the meat has probably already been consumed and no illnesses have been reported.

"That's the truly nice thing," he said. "We've recalled all ground beef from all stores, even though there's only a suggestion that there's a problem with one store, but we want to do everything possible to protect the public."

Read said WinCo is cooperating with the investigation by California and federal officials.

Bill Marler represented more than 100 victims in the 1993 Jack in the Box E. coli outbreak that killed four children. He commended WinCo Foods for issuing the recall, given that the information came from a private study.

"They certainly could have pushed back and said, 'What is this, it's a lawyer doing testing in a lab in Seattle," he said. "They could have taken the much less pro-consumer point of view, because obviously this is not something a company wants to do. Under the circumstances, I certainly appreciate what they did."

E. coli is a potentially deadly germ that can cause bloody diarrhea, fever, vomiting and, in severe cases, kidney failure.