Federal officials say the biggest beef recall in history is motivated by caution and uncertainty.

The USDA says a California slaughterhouse killed cattle too sick or injured to walk on their own, which is in clear violation of federal food inspection laws.

Federal inspectors will never know what afflicted the cattle, but they're recalling millions of pounds of beef just to be safe.

This story is as troubling for what happened as for what did not. An undercover video shows cows too sick or injured to stand being abused at a southern California slaughterhouse before being slipped past government inspectors into the food chain.

It provoked outrage and the historic recall of 143 million pounds of beef produced over a two year period. The vast majority of it has obviously has already been consumed, but the government said something had to be done because of the egregious violations by employees at the Westland-Hallmark Meat Company.

"It's hard to believe that this kind of handling of animals happens anywhere in the United States, much less at a federal slaughter establishment," Food Safety & Inspection Service – USDA Dr. Kenneth Peterson said.

Westland-Hallmark provided beef for fast-food chains and the federal school lunch program. Officials say it's unlikely the remaining beef is tainted and no illness has been reported, but it wasn't properly inspected, and therefore, is unfit for human consumption.

Now local authorities are scrambling to find the remaining beef and ease fears. The cattle industry calls this an aberration.

"While this company failed to execute one of the regulations in place and it's not acceptable, there are a number of interlocking safeguards in place that make sure our beef is safe when it comes to our homes," Kim Essex with the National Cattlemen's Beef Association said.

Critics say this also raises concerns about government inspections.

"Food safety laws today are a hundred years old,” Caroline Smith DeWaal with the Center for Science in the Public Interest said.

It’s a historic recall and new questions about a weak link in the food chain that took two years to find. Critics say this is a red flag for the nation's food inspection system and proof Congress needs to toughen the laws.