Washington ? The Agriculture Department is ending its search for additional cases of mad cow disease even though officials have not found several animals suspected of having eaten the potentially infectious feed believed to have caused the only known U.S. case.

“Our investigation is now complete,” Dr. Ron DeHaven, the department’s chief veterinarian, said Monday. “We feel very confident the remaining animals, the ones we have not been able to positively identify, represent little risk.”

The closure leaves officials not knowing what happened to 11 head of cattle among 25 that authorities say were most likely to have eaten the same feed as that given to a Holstein diagnosed in Washington state with mad cow after it was slaughtered on Dec. 9.

All 25 were among 81 born on a farm in Alberta, Canada, and shipped into the United States in 2001. Officials have found 29 of the 81, including 14 considered most at risk.

The search for the 81 cattle led authorities to 189 farms and ranches and the testing of 255 animals, none of which had bovine spongiform encephalopathy, or BSE, the technical name for mad cow disease, DeHaven said. Some may have gone to slaughter, but BSE tests would have spotted any slaughtered animal that had mad cow, he said.

The likelihood of finding more cases “is pretty slim at this point,” DeHaven told reporters during a telephone conference.

An international review panel created by Agriculture Secretary Ann Veneman said last week that U.S. officials had done a thorough job of searching for the animals but added that more focus should be put on preventing future cases.

The government also has found 2,000 tons of rendered protein into which tissue from the Holstein could have been mixed, said Dr. Stephen Sundlof, director of the Food and Drug Administration’s Center for Veterinary Medicine, which regulates animal feed.

The material is being destroyed so none can make its way into animal feed, Sundlof said. Experts say meat and bone meal from infected animals can transmit mad cow’s infectious protein to other animals.