A salmonella outbreak that has sickened 810 Americans since early April is still ongoing, and may not be linked to tomatoes after all, federal health authorities said Friday.

Tomatoes still have the strongest association with the outbreak, said Patricia Griffin, of the Centers for Disease Control.

"We don't have any evidence that whatever the source is, it's been removed from the market," said Griffin, chief of the CDC's Enteric Diseases Epidemiology Branch, during a conference call with reporters.

But investigators are looking at other foods that might have been eaten along with raw tomatoes, especially produce items in foods commonly consumed by outbreak victims, such as pico de gallo, guacamole and fresh salsa.

Consumers were still getting sick as recently as June 15, just 12 days ago. On average, it takes 16 days to get laboratory confirmation of a food borne illness, Griffin said. That means it's too soon to tell if the outbreak is winding down.

For now, the U.S. Food and Drug Administration has not changed its advice about tomatoes. The FDA says that cherry and grape tomatoes and tomatoes on the vine are safe to eat, and that it's also OK to eat round red tomatoes and Roma, or plum tomatoes, from approved growing regions.

The FDA has cleared areas that weren't producing tomatoes when the outbreak began on April 10, including Georgia. Tomatoes from some of those regions are traveling through packing houses and distribution centers that the FDA is checking as part of its attempts to trace the contaminated tomatoes, said David Acheson, the agency's associate commissioner for foods.

In the past week, investigators have taken 1,700 samples from farms, packing houses and warehouses in Mexico, Florida and other parts of the United States, looking for the Salmonella Saintpaul strain causing the illnesses. So far, no bacteria has been found.

In Georgia, the number of laboratory-confirmed illnesses has grown to 18.