BILLINGS -- Federal and state officials continued Thursday to "trace back" cattle that may have come into contact with seven cows near Bridger that have tested positive for brucellosis.

The U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) and the Montana Department of Livestock are looking for any cattle from a second herd that might test positive. If even one animal in a herd separate from the Bridger group fails the test, Montana will lose its brucellosis-free status and its livestock industry will be saddled with expensive testing and other requirements.

APHIS spokeswoman Teresa Howes of Fort Collins, Colo., said investigators are interviewing producers in the Bridger area this Memorial Day weekend. Under APHIS rules, a state has 60 days to conduct the investigation.

Montana's clock started May 18 when Gov. Brian Schweitzer announced the positive tests, so the deadline is July 6. However, Howes said that Montana Acting State Veterinarian Jeanne Rankin can request more time through a waiver petition.

"She would do that through APHIS and sooner is always better," Howes said.

On Wednesday, the North Dakota State Board of Animal Health ordered most breeding-age Montana cattle to be tested for brucellosis and to meet other health requirements before entering North Dakota.

North Dakota State Veterinarian Susan Keller said she's unaware of other states surrounding Montana taking similar actions.

"During the gray period while they still are doing the testing and the animals continue to move, we're going to take this precaution," Keller said. "It may take 60 days and a lot of movement can occur between our states in that time."

Under the North Dakota order, cattle grazing in counties bordering both states may not need testing because they can move between the states. Keller said producers should check the rules with her office. The telephone number is 701-328-2655.