INSERT DESCRIPTIONIt’s like finding the Salmonella in a tomato stack writ large. (Alan Zale for The New York Times.)

At the outset of the Food and Drug Administration’s conference call last Friday on the Salmonella outbreak, officials made two points.

– People are still getting sick as of June 15. Since April 23, when the agency first detected a problem, 810 people have fallen ill in 32 states.

– There’s no sign of the source of the Salmonella. The investigation has sent teams to Florida, Mexico and other key points in America’s food chain, but they have turned up zilch.

On that point, Dr. David Acheson, the agency’s associate commissioner for foods, was utterly frank:

Total number of samples that have been collected so far are 1,700, approximately. And this is a mixture of domestic and imported samples. Many of them are imported samples.

And to date, every single one is negative. I’ll just repeat that. We’ve collected approximately 1,700 samples. This has been, again, a joint effort between different parts of the federal government, including FDA and USDA. And several states were also testing tomatoes for the Salmonella. And so far, all of these have been negative.

As shocking as the 0-for-1,700 statistic might seem, Mr. Acheson was not fazed. He had warned about two weeks ago that “we may not ultimately know the farm where these came from.” And on Friday he reiterated the point. “That’s not that unusual with tomato outbreaks,” he said. “It’s not that infrequent for us to be unable to actually trace back specifically to the source.”

The mystery remains unsolved for a number of reasons, but the agency is identifying one chief culprit: “this repacking phenomena.” Mr. Acheson was referring to the practice by many suppliers — perhaps 90 percent he suggested — who take shipments of tomatoes from multiple farms that could be located in different countries and then mix them according to orders. Since tomatoes aren’t required to have identifying marks on their skin, the trail goes cold at that point in the chain.

“It’s not exactly a lost cause,” Mr. Acheson said, “but as I said, the colder the trail gets, the less likely that you’re actually going to find the problem.”

As sicknesses continue beyond the shelf life of tomatoes — it’s been almost three months since the first Salmonella case — officials are also starting to question whether another vegetable is to blame, according to this morning’s USA Today.

“We’re broadening the investigation to be sure it encompasses food items that are commonly consumed with tomatoes,” Robert Tauxe of the Centers for Disease Control and Prevention said. Several state health officials echoed those concerns in comments to The Baltimore Sun today:

“It’s bad, and I think everyone will be very apologetic” if it turns out tomatoes weren’t the source, said Tim Jones, Tennessee’s state epidemiologist, describing himself as “increasingly concerned” about whether tomatoes are to blame.

But apologies won’t erase the losses in the food industry. The National Restaurant Association, a leading trade group, estimated the losses at about $100 million, according to The Wall Street Journal. And the tomato recall is hitting restaurants as they struggle to attract American consumers who are short on cash.

The C.D.C. said that many people who fell ill had eaten tomatoes in salsa and guacamole, so any ingredients in those may be to blame: jalapeƱo peppers, green onions, cilantro and so on. Meanwhile, markets in Houston and other cities are beginning to cautiously restock tomatoes.

As the mystery continues, Mexican food enthusiasts are enduring a purgatory of sorts. While tomatoes may be making them sick, the cuisine can be pretty bad without them as well. One restaurant owner in Austin told the Journal that his attempt at a tomato-less pico de gallo was less than successful. “It was just green and white,” Tony Villegas said. “It tasted really bad, unless you really like onions.”