Since April, 1,017 people in 41 states and Canada have been diagnosed with infections of Salmonella Saintpaul with the same genetic fingerprint, said the Centre for Disease Control and Prevention.
"The accumulated data from all investigations indicate that jalapeno peppers caused some illnesses but that they do not explain all illnesses," the CDC said in a statement.
Raw tomato also a suspect
"Raw tomatoes, fresh serrano peppers, and fresh cilantro also remain under investigation."
The CDC and US Food and Drug Administration said warnings about eating tomatoes remained in place. They include raw red plum, red Roma, or red round tomatoes.
"Until health officials know that the contaminated product or products are no longer on the market, persons with increased risk of severe infection, including infants, elderly persons, and those with impaired immune systems, should not eat raw jalapeno peppers or raw serrano peppers," the CDC added.
The investigation has been challenging, the CDC said. "One difficult aspect is that people often have difficulty remembering exactly what foods they ate, and remembering specific ingredients in those foods is even more difficult," it said.
"Perishable foods that were consumed by ill persons are often not available to test. When food items are mixed together and consumed in the same dish, all the items may be statistically linked to illness."
Salmonella causes diarrhoea, fever and abdominal cramps 12 to 72 hours after infection.