Minneapolis ? Preliminary tests show three pigs in Minnesota may have contracted the swine flu virus making them the first potential U.S. cases in swine, agricultural officials said Friday. They stressed the finding does not threaten food safety.

The samples were taken from pigs shown at the Minnesota State Fair between Aug. 26 and Sept. 1 as part of a university research project. Officials expect results next week to confirm whether the pigs were infected with swine flu virus, also known as H1N1.

The pigs did not show signs of sickness and officials said they likely contracted the virus from some of the nearly 1.8 million people who visited the fair.

The Department of Agriculture’s veterinary lab in Ames, Iowa, is conducting tests to confirm the results, Agriculture Secretary Tom Vilsack said in a statement.

In a conference call with reporters Friday, Minnesota Agriculture Commissioner Gene Hugoson said officials don’t know what happened to the three pigs, but that they probably were sent to slaughter soon after they were shown at the fair, which ended on Labor Day.

“This is not an issue of food safety,” Hugoson said. “Pork products are and continue to be safe to eat.”

Authorities aren’t planning special measures if the tests confirm the Minnesota pigs had the virus. Hugoson said farmers will continue watching herds for flu symptoms, and slaughterhouse inspectors will continue rejecting pigs showing disease symptoms.

Agriculture officials had expected the virus to reach domestic pigs this year and have guidelines to slow its spread, said Jeff Bender, director of the University of Minnesota’s Center for Animal Health and Food Safety. Herd infections previously were reported in Canada, Australia, Argentina, Ireland, the United Kingdom and Norway. A hog vaccine is being developed but isn’t yet available.

Jose Diez, a USDA veterinary official on Friday’s call, said killing pigs to stop the spread of the virus has not been considered.