Health officials in Iowa and Nebraska on Tuesday identified prepackaged salad mix as the source of a severe stomach bug that sickened hundreds of people in both states, but federal authorities said it's not clear whether cyclospora outbreaks elsewhere in the U.S. are also linked to that produce.

Cyclospora is a rare parasite that causes a lengthy gastrointestinal illness, and outbreaks of the illness have been reported in 15 states. The U.S. Centers for Disease Control and Prevention said Tuesday that it's not clear whether all of the illnesses are linked to a single source. The outbreak has sickened at least 145 residents in Iowa and 78 in Nebraska.

Nebraska officials said the salad mix in question included iceberg and romaine lettuce, along with red cabbage and carrots, which came through national distribution chains. They did not identify specific brands. A Nebraska health department spokeswoman said the agency was working with the U.S. Food and Drug Administration to get a "clear picture" of which were involved and whether they're tied to one common source, such as the same farm or producer.

"Our goal is to protect Nebraskans, pinpoint the source of the illness and make sure the risk is eliminated," said Dr. Joseph Acierno, the department's chief medical officer and director of public health.

In Iowa, officials said they were confident that most if not all of the product was no longer on the shelves. The affected products were traced to grocery stores and restaurants, said Steven Mandernach, the state's top food-safety inspector. Mandernach said cases were reported throughout the state, but the largest number was in the eastern Iowa city of Cedar Rapids.

Mandernach said officials have traced 80 percent of the Iowa cases to a common source, which he did not identify because officials believe there's no longer any immediate safety threat. Mandernach said it's possible that the parasite spread through contaminated floodwater and onto farm fields after arriving in the state. Before the outbreak, he said, Iowa had seen about 20 cases of cyclospora in the last decade.

Local health departments are working with the U.S. Food and Drug Administration to identify exactly where the contamination originated in the food production chain and where the product was distributed.