Was it the cilantro? The tomatoes? Maybe some bad meat? The culprit behind a multi-state foodborne illness outbreak linked to Chipotle Mexican Grill restaurants in the Northwest may never be identified. Despite closely tracking the outbreak, interviewing those sickened, and testing a variety of food samples, local and federal health experts have so far come up empty-handed in the search for its source. With no trace of the source in sight, Chipotle announced on Tuesday that it is moving to reopen affected stores.

Meanwhile, patients' test results continue to roll in from the unresolved outbreak, which spread Shiga toxin-producing Escherichia coli O26 (STEC 026) to unsuspecting consumers. The count as of Wednesday is 46 sickened people, with 27 in Washington state and 19 in Oregon. STEC 026 causes bloody diarrhea, abdominal cramps, vomiting, and fever and can lead to death (none have been reported in this outbreak). Though the source is unknown, many of those sickened reported eating at a Chipotle restaurant.

Overall, the outbreak highlights an all too common dichotomy of foodborne illnesses in the US: experts are getting better at detecting and tracking outbreaks, but they still struggle to solve and prevent them—and in some ways, it’s getting harder.

In recent decades, the Centers for Disease Control and Prevention have stepped up investigations and monitoring of foodborne illnesses, including setting up a network of databases that collect genetic fingerprints of foodborne pathogens. Those improvements help explain increased reports of such outbreaks, particularly outbreaks that affect people in more than one state, CDC experts note. For instance, from 1973 to 1980, the agency documented about 2.5 multi-state outbreaks a year, from 2000 to 2010 the number jumped to 13.5, and between 2010 and 2014 it was about 24.

Of course, it could also be that there are just more multi-state outbreaks now than in the past, possibly due to increasingly large-scale and complex food supply chains. But with poor surveillance in the past, it’s unclear. What is clear, however, is that recent multi-state outbreaks have been more deadly and have involved more harmful pathogens than those of single-state outbreaks.

Of the 4,163 foodborne illness outbreaks between 2010 and 2014, only 120, or three percent, were multi-state outbreaks. But those multi-state outbreaks accounted for 56 percent of deaths linked to foodborne illnesses. The leading pathogens in multi-state outbreaks included Salmonella, STEC (as was the case in the Chipotle outbreak), and Listeria monocytogenes, all of which can cause severe disease and even death. The leading pathogen linked to single-state outbreaks was norovirus, which tends to produce relatively mild illness.

That makes multi-state outbreaks more concerning. Yet they can be harder to sort out. “Rapid identification of the food that caused the outbreak, discovering where the contamination occurred along a complex supply chain, and recalling a food distributed across the country and perhaps around the world are challenging tasks,” CDC officials noted in the report. Such a feat requires fast access to scientific technology, such as genome sequencers, as well as coordination with local health agencies and retailers. And relying on sick people to accurately remember what they ate is also a challenge. CDC experts recommend things like using retailer “loyalty” cards to track food purchases and identifying clusters of sick people, which could offer clues.

However, in the case of the Chipotle outbreak, a leading barrier to tracking the source of infection might be an innate feature of fast food—it goes fast. “Food outbreak investigations do not always identify a specific food source. A common reason for this is that the contaminated food source was consumed before the food samples were collected,” the Washington State Department of Health noted.

With the source of the STEC 026 possibly gone without a trace, Chipotle is gearing up to reopen all 43 closed stores “in the coming days.” The restaurant chain says it has replaced all the food in the affected restaurants, and it had 900 tests come back negative for STEC. It will also start testing food and equipment, improving sanitizing and handling procedures, and upping food safety auditing in all of its restaurants.