The U.S. Department of Agriculture alleged for the first time Thursday that a Petaluma slaughterhouse engaged in "circumvention" of federal inspection rules.

The allegation — strongly denied by a partner in the business — came in a statement responding to repeated questions from the media and food safety experts, asking how so many cattle last year failed to receive a full inspection at Rancho Feeding Corporation's facility. The USDA's official answer suggested a distinction between an accidental breach in slaughterhouse protocol and intentional wrongdoing.

In the statement, the department's Food Safety and Inspection Service said its "inspectors were present at Rancho Feeds during normal operations as required by law. The ongoing investigation is associated with the company's intermittent circumvention of inspection requirements."

The USDA inspector general is conducting a separate investigation into the plant.

While minimal, Thursday's 27-word statement was the first time the USDA has elaborated on the focus of its investigation since Feb. 8, when it announced the recall of 8.7 million pounds of beef sold in the United States and Canada. At that time, the USDA asserted that Rancho "processed diseased animals" without a full inspection.

"Somebody in authority looked at this scenario and said, there's been meat going through the system that shouldn't," said Bill Marler, a prominent food safety attorney in Seattle who is not directly involved in the matter.

The USDA has not received any reports of illness linked to the meat, an agency spokesman said Wednesday.

The government's allegation was strongly denied by Robert Singleton, one of Rancho's owners.

Singleton, who on Thursday made his first public comments on the investigation, acknowledged there were days each week when he wasn't at the property. But he insisted Rancho consistently slaughtered animals in accordance with the USDA's rules.

"There was always an inspector on the property," he said. "We never harvested without an inspector on site."

Experts said the USDA's statement suggested exactly the opposite.

"It could mean that perhaps animals had been railed out for further inspection and that wasn't complete yet and when an inspector wasn't present those animals were processed," said David Theno, CEO of Gray Dog Partners, a Del Mar food industry and safety consulting firm.

Theno was formerly a senior vice president and chief food safety officer for Jack in the Box, Inc., which he joined after it was enveloped in scandal when its burgers were blamed for a massive outbreak of food poisoning.

"The agency virtually never takes this kind of action unless they've got pretty solid information to act on. This is big time stuff," he said.