Since the mid-1980s, scientists have identified dairy cows as the primary reservoir of Salmonella Newport. A closer look at established facts points to an ongoing food safety crisis hidden in plain sight.

Still, major questions remain. The United States Department of Agriculture’s Food Safety and Inspection Service (FSIS) may again broaden the scope of the recall, as it already did on Thursday. More stores may be added to the list of affected retailers published over the weekend. And, of course, more Americans may continue to fall ill. But while basic facts—how much meat, from which stores, causing how many illnesses—remain unclear, a larger uncertainty looms. Namely: How does nearly 7 million pounds of beef get exposed to Salmonella in the first place, then get shipped out to the public? What, exactly, went wrong at Tolleson?

Late last week, JBS, the world’s largest meatpacker, recalled 6.9 million pounds of ground beef that it said may have been tainted with Salmonella Newport. Here’s what we know four days into the recall: the strain is responsible for sickening 57 people in 16 states. All of the meat came from the same JBS plant in Tolleson, Arizona. And in less than a week, the incident has already reached historic proportions. It’s the largest recall of beef since the notorious Rancho Feeding Inc. recall of 2014 . Former USDA food safety specialist Carl Custer has said it’s largest- ever recall of ground beef related to Salmonella.

Update, December 4, 2018 at 2:45 p.m., EST: The United States Department of Agriculture’s Food Safety and Inspection Service has expanded its October 4 recall by five million pounds, bringing the total to nearly 12 million pounds of ground beef. The Centers for Disease Control (CDC) has revised its count of people sickened by meat recalled from JBS’s plant in Tolleson, Arizona. The agency now confirms 246 illnesses in 24 states.

When I asked FSIS for additional insight, I was told I’d have to file a Freedom of Information Act (FOIA) request to learn more. JBS did not respond to multiple requests for comment. So far as the official channels are concerned, we’re still largely in the dark.

And yet, the few details voluntarily released are very revealing if you read between the lines, helping to explain why the meat of an estimated 13,000 animals, a small city of cattle, is now headed for the landfill.

The people I spoke to for this story suggest this outbreak had a clear origin point: a dairy farm in the Southwest. That’s important, because dairy cows processed for meat turn out to be a kind of food safety blind spot. For reasons I’ll explain, dairy cows sickened by Salmonella are more likely than healthy ones to be sent to meat plants for slaughter. Once there, they’re likely to be ground up and used as filler in thousands of pounds of beef, dramatically increasing their risk potential. Perhaps most surprisingly, there’s no system in place to track or disarm this risk. In fact, thanks to a quirk in food safety law, meatpackers aren’t required to test for Salmonella. And even when it is present, the government can’t really do anything about it—not even if millions of pounds of tainted product are at stake.

While we may never know the exact details of this outbreak, we can look to previous recalls for clues—and established facts point to a massive, ongoing food safety crisis hidden in plain sight.

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Tolleson, Arizona, situated just west of the Phoenix metropolitan area, is surrounded by cows.

Arizona is the 13th highest milk-producing state by volume. Neighboring New Mexico, with 323,000 cows producing more than 8 billion pounds of milk in 2017, ranks in the top ten. But in the realm of livestock transport, where farmers routinely have to drive their animals hundreds of miles to be slaughtered, Tolleson is less than a day’s drive from the country’s most productive dairy region: central and Southern California.