Toronto ? Test results on three more cattle herds linked to a lone case of mad cow disease in Canada came back negative Friday, and a government inspector said the investigation into the infection could be over by the end of next week.

Brian Evans, chief veterinary officer of the Canadian Food Inspection Agency, said two herds that received calves from the infected cow’s last herd tested negative for bovine spongiform encephalopathy, or BSE.

Another herd where the infected cow lived for up to four years also tested negative, he said.

Animals must be killed to be tested, and 370 animals, including the 192 in the infected cow’s last herd, have been slaughtered for testing so far, with no further cases found. More than 600 animals must still be tested.

More than a dozen herds are under quarantine for the investigation. Officials are trying to find out where the infected cow came from and what feed it ate.

Several countries, including the United States, banned all Canadian beef products after officials announced May 20 that one BSE case had been detected in Alberta, the heartland of Canada’s cattle country. It was the first reported BSE case in North America in a decade, and only the second ever on the continent.

Canada and the United States have discussed relaxing the U.S ban, which closed the main market for Canadian beef, even while the investigation continues.

Mad cow disease was first diagnosed in Britain in 1986.