Officials did not offer additional information about the animal, according to media reports. Tests on a second carcass, also singled out in preliminary testing as possibly being infected, was pending and won't be available for several days.
"The USDA remains confident in the safety of America's food supply," John Clifford, deputy administrator of the department's Animal and Plant Health Inspection Service, said in a statement.
Clifford indicated that tests by the federal laboratory at Ames, Iowa, on tissue from the carcass screened last Friday were negative.
No further details such as what type of animal, where the animal came from or what lab did the (initial) testing would be disclosed, Clifford said.
Clifford defended the department's practice of disclosing initial screening results even when they are inconclusive and are likely to be discounted, as was the case with this animal when more precise chemical tests are conducted.
The preliminary test last Friday was the first time in 8,587 such screening tests where the results came up "inconclusive," raising the possibility of a mad cow disease infection since the screening began June 1. Another inconclusive test on a second carcass was announced Tuesday. Clifford said the follow-up tests on those tissue samples hadn't yet been completed.