LINCOLN, Neb. – Nearly 400 people across the country have been sickened by cyclospora, a lengthy intestinal illness usually contracted by eating contaminated food. But if you’re looking to find out exactly where it came from, you may be out of luck.

Federal officials warned Wednesday that it was too early to say whether the outbreak of the rare parasite reported in at least 15 states was over. No cases were reported in Maine.

Health officials in Nebraska and Iowa say they’ve traced cases there to prepackaged salad. They haven’t revealed the company that packaged the salad or where it was sold, explaining only that most if not all of it wasn’t grown locally.

The lack of information has fueled concern from consumers and food safety advocates who argue that companies should be held accountable when outbreaks happen and customers need the information about where outbreaks came from to make smart food choices.

“If you want the free market to work properly, then you need to let people have the information they need to make informed decisions,” said Bill Marler, a Seattle attorney who specializes in class-action food-safety lawsuits.

Mark Hutson, who owns a Save-Mart grocery story in Lincoln, Neb., said he was unaware of customers who had raised concern about the product, which was unusual in situations involving foodborne illnesses. But Hutson said the lack of specific brand information threatened to hurt all providers, including the good actors who did nothing wrong.

“I think there was so little information as to what was causing the problem, that people just weren’t sure what to do,” he said. “Frankly, we would prefer to have the names out there.”

Authorities said they still hadn’t determined whether the cases of cyclospora in the different states are connected.

“It’s too early to say for sure whether it’s over, and thus too early to say there’s no risk of still getting sick,” said Maria-Belen Moran, spokeswoman for the Centers for Disease Control and Prevention.

Only Iowa and Nebraska officials had directly linked the outbreak in their states to a salad mix of iceberg and romaine lettuce, carrots and red cabbage. But grocery shoppers elsewhere acknowledged it was a factor as they shopped for produce.

“I can’t say I really want to go and buy particularly any lettuce right now,” said Laura Flanagan, 35, who was shopping at a Whole Foods in Dallas with her two young children. “I’m being pretty cautious about it.”

The Food and Drug Administration said Wednesday that it didn’t have enough information to name a possible source of the outbreak. In the past, the agencies have at times declined to ever name a source of an outbreak, referring to “Restaurant A” or using vague terms.