Health officials broadened their search today through nine states for chickens laying eggs poisoned with PCB. But they remained at a loss to explain how the chemical got into the chickens' feed in the first place.

As the probe spread, farmers in Idaho and Utah continued the task of destroying more than 400,000 contaminated chickens and more than one million poisoned eggs. Some meat-processing plants and bakeries were forced to interrupt production.

The chickens are being gassed and the bodies burned. The eggs are being buried.

Most of the poisoned chickens came from Ritewood Egg Co., in the southeastern Idaho town of Franklin, which ships chickens, baby chicks and eggs to most of the western and Rocky Mountain states. The company may have sold more than 18 million contaminated eggs since the PCB was detected two weeks ago.

Idaho Gov. John Evans criticized the U.S. Food and Drug Administration today for not telling the public immediately when the PCB was first discovered. The chemical was first found in meat products processed at the Butterfield Foods plant in Butterfield, Minn., and at the Swift & Co. plant in Clinton, Idaho.

"The eggs should have been pulled off the shelves immediately," said the annoyed Evans. "There was a communications breakdown between the FDA and our state health officials that was inexcusable. If Idaho had known, we would have taken steps to do something about it."

The FDA, which is conducting the search along with state health officials in Idaho, Montana, Utah, Wyoming, Washington, Oregon, Minnesota and North and South Dakota, said the chances of anyone becoming ill because of the PCBs were very slim.

However, PCBs and their derivatives are becoming widely dispersed throughout the environment, and the long-term impact of that is not well understood.

Scientists at the American Chemical Society meeting in Washington this week reported that literally tons of the chemical fall out of the air into the Great Lakes every year, and that it has been detected in sperm of college students in Florida. Some 23 percent of the students are functionally sterile, and researchers blame that, in part, on the PCBs.

The FDA said the PCB was detected immediately at the Butterfield and Swift plants where it was identified in chicken meat about to be used to make Campbell's soup and chicken pot pies. The chemical was also identified at an Oregon bakery which bought 50,000 of the poisoned eggs.

Officials said the contaminated eggs contained more than 10 times the amount of allowable PCB. But Utah's state health director, Dr. James Mason, said that a 35-pound child would have to have eaten "three dozen eggs a day for 60 days" to become ill.

Manufacture of the polychlorinated biphenyl, or PCB, was stopped by the Environmental Protection Agency in 1976 when it was found to cause cancer and birth defects in laboratory animals, and skin lesions and liver problems in humans.

The chemical was widely used as a high-temperature lubricant in engines and as a heat retardant in electrical components.

Leroy Gomez, FDA Rocky Mountain regional director, said in Denver today that while investigators believed the PCB came from chicken feed sold to the Ritewood Co. by the Pierce Packing Co., a Billings, Mont., pork processor, they were not sure how the chemical got there.

"If it wasn't in the feed at the start, we'll have to look at the people who brokered the feed and the people who used it," said Gomez.

Pierce president Ken Poggi said he believed his firm would be cleared as the source by an FDA team that flew to Billings Thursday. He said he thought the source of the PCB would be identified as a trucker who picked up the feed, made of hog slaughter by-products, from his loading dock.

But Gomez said that an explosion in electrical equipment at the Pierce plant a few weeks ago may provide clues for the poisoning. The plant sells 100,000 to 200,000 pounds a week of meal made with meat to feed brokers across the nation.

The number of people who may have eaten eggs or chicken meat products poisoned with PCB, an Idaho state official said, could run into the "hundreds of thousands." He said the chances of anyone showing PCB poisoning symptoms "were next to impossible."

FDA officials said that if their continuing search shows any high levels of PCB, more eggs, chickens and meat and bakery products could be destroyed. "The chances of finding more contamination at chicken farms in the western states is good," said the Idaho official.