WASHINGTON—As salmonella cases continue to climb, the government is checking if tainted tomatoes really are to blame for the record outbreak—or if the problem is with another ingredient, or a warehouse that is contaminating newly harvested tomatoes.

The widening outbreak—with 810 people confirmed ill—means whatever is making people sick could very well still be on the market, federal health officials warned on Friday.

Tomatoes remain the top suspect and the advice on which ones consumers should avoid hasn’t changed, stressed Food and Drug Administration food safety chief Dr. David Acheson.

However, he said it is possible that tomatoes being harvested in states considered safe could be picking up salmonella germs in packing sheds, warehouses or other facilities currently under investigation.

Most worrisome, the latest victim became sick on June 15—long after the outbreak began on April 10 and weeks after government warnings stripped supermarkets and restaurants of many tomatoes.

“The source of contamination has been ongoing at least through early June. And we don’t have any evidence that whatever the source is, it’s been removed from the market,” said Dr. Patricia Griffin of the Centers for Disease Control and Prevention.

Disease detectives at the CDC in Atlanta are double-checking their own probes just in case some other type of produce is really the culprit.

She wouldn’t identify other potential suspects, except to say that from the beginning some patients have told the CDC they ate raw tomatoes in fresh salsa and guacamole. Officials have previously cleared jarred salsas.

For now, the FDA continues to urge consumers nationwide to avoid raw red plum, red Roma or red round tomatoes unless they were grown in specific states or countries that FDA has cleared of suspicion. Check FDA’s Website, www.fda.gov, for an updated list. Also safe are grape tomatoes, cherry tomatoes and tomatoes sold with the vine still attached.

But FDA’s Acheson made clear that consumers should stay tuned in case that advice changes.