WASHINGTON – A salmonella outbreak that sickened dozens of people in Maine and 18 other states appears to be over, while investigators remain unsure how it began, a federal official said Tuesday.

Health officials believe the outbreak may be linked to tomatoes or other fresh produce contaminated with the bacteria, but have been unable to pinpoint a specific product or source. There have been no deaths, but 171 people have fallen sick. Officials revised downward the tally by one case Tuesday after further investigation.

Maine reported eight cases, five involving children, and all originating in Cumberland and York counties. All eight people have fully recovered and only one was hospitalized.

Reports of illness peaked in late September, suggesting the outbreak is now over, said Dr. David Acheson, chief medical officer in the Food and Drug Administration’s Center for Food Safety and Applied Nutrition. There is little evidence of any ongoing risk, he added.

“Whatever was contaminated that caused the illness, it has either been consumed, destroyed or thrown out. So the suggestion there is a need to put out a consumer warning about produce on the shelf is unwarranted. It seems to be past,” Acheson said.

The Centers for Disease Control and Prevention continue to interview people sickened in the outbreak, but the information gathered is too vague or broad for the FDA to trace the contamination to its source, Acheson said.

In the case of the recent E. coli outbreak, which killed three people and sickened more than 200, investigators were able to quickly identify fresh, bagged spinach as the culprit. The information on the bags – including brand name, sell-by date and lot number – allowed them to then zero in on the likely source of the tainted spinach: a handful of farms in California’s Salinas Valley.

If fresh tomatoes were the source of the salmonella outbreak, that sort of detective work is more difficult, Acheson said.

“You can get a lot of information from looking at a bag. You don’t get that information from looking at a tomato,” Acheson said.