June 27 (Bloomberg) -- The U.S. is examining whether something other than tomatoes may be the source of a salmonella outbreak that has now sickened 810 people.

While tomatoes remain the likely cause for the growing toll of illnesses reported, investigators can't rule out that another food is to blame, said Patricia Griffin, of the U.S. Centers for Disease Control and Prevention in Atlanta, on a conference call today with reporters.

``There's a strong epidemiologic association with tomatoes,'' Griffin said. ``We have also kept an open mind about other possibilities and are looking into other ingredients. And when we know anything we will certainly let people know.''

The number of people who have become ill is a record for salmonella cases associated with produce, according to the CDC, and is about four times the 205 cases reported after packaged fresh spinach was tainted by E. coli in 2006. The tally of reported illnesses described by the CDC increased by 54 since yesterday alone.

The outbreak began in April, and the most recent case involved a person who said they became ill on June 15, according to the CDC. The outbreak, linked to illnesses in 36 states and the District of Columbia, is ongoing and the CDC doesn't know whether the number of cases has peaked, Griffin said. There is often a lag of more than two weeks before cases are confirmed.

Store Shelves

Investigators can't rule out that tainted products are still on store shelves or that tomatoes are continuing to be contaminated in a packing house or some other point in the distribution chain, said David Acheson, the Food and Drug Administration's associate commissioner for foods, during the conference call.

FDA officials have said they believe the outbreak involves tomatoes grown in Mexico or Florida, and investigators are checking points along the distribution chain leading from those locations to pinpoint the source of contamination. Samples of tomatoes taken during the investigation have all tested negative, Acheson said.

Acheson said FDA may never find the cause of the outbreak, repeating a possibility he raised previously. The task becomes more difficult as the trail grows ``cold,'' he said.

``It's possible that this investigation will not ultimately provide a smoking gun that enables us to pinpoint the source of the contamination,'' Acheson said.

The FDA is continuing to check ``up and down the chain'' trying to find the source, he said. Investigators have identified ``clusters'' of illnesses from certain regions and are trying to use those cases to trace the cause, according to the FDA.

Packaged Together

Tomatoes have been shipped from Mexico to Florida, combined with Florida tomatoes, and packaged together, complicating the FDA's investigation, Acheson said. In other cases, tomatoes were shipped from Florida to Mexico for packaging, and then shipped back to the U.S.

Cherry, grape, vine and home-grown tomatoes from all regions remain safe to eat, according to the FDA. Before buying raw red plum, Roma or round tomatoes, consumers should ask where the products were grown, according to the FDA.

If the tomatoes are from locations listed as cleared on the FDA's Web site, the products are safe to eat. If they aren't on the list or vendors don't know where they're from, the tomatoes shouldn't be consumed, the FDA says.

Tomatoes from states that have been cleared could still be contaminated in a warehouse or another point in the distribution chain, Acheson said.

Most people infected with salmonella have diarrhea, fever and cramping for four to seven days and recover without treatment. Infants, the elderly and people with impaired immune systems may develop severe cases that lead to hospitalization or death.

To contact the reporter on this story: Justin Blum in Washington at jblum4@bloomberg.net.

Last Updated: June 27, 2008 17:58 EDT