TUESDAY, June 17 (HealthDay News) -- Although a U.S. government investigation into salmonella-tainted tomatoes has not yet identified a specific source for the contamination, health officials said Monday they are now focusing their "trace-back" efforts on one cluster of nine cases in one location.

"The cluster is linked to the same geographic location, and all [victims] are appearing to have consumed similar types of tomatoes," Dr. David Acheson, associate commissioner for food protection at the U.S. Food and Drug Administration, told reporters at a teleconference. "We're not there yet on the cluster, but I'm hopeful that this is our most fruitful lead to date on the trace-back."

Officials would not divulge the location of the cluster or comment on whether the cluster was the same one reported in an e-mail by a top FDA official on Friday. That cluster involved nine victims who ate at two restaurants in the same chain, which health officials refused to name.

Meanwhile, officials at the U.S. Centers for Disease Control and Prevention said the number of people sickened in the outbreak now stands at 277, an increase of 49 over last week's count, with 43 hospitalized. And the number of states reporting illnesses also increased by 5 plus Washington, D.C.

Health officials continued to say the outbreak appears to be diverse in its origins.

"There is no one chain of restaurants or supermarkets or retail stores that ties this all together," said Dr. Ian Williams, chief of the CDC's OutbreakNet Team. "We have seen it from people who have consumed [tomatoes] in homes, in restaurants; no one individual or grocery chain accounts for all of the cases. With that said, there have been clusters noticed such as the one we're discussing right now."

Williams added that the outbreak is still considered "ongoing."

On Friday, the FDA said the bulk of the tomatoes available in the United States at the start of the outbreak in April came from Mexico and sections of Florida.

Tomatoes currently being harvested in Florida are coming from the north and are coming to market with a certificate from the state guaranteeing that they were harvested in that area and are safe to eat, Acheson said. Many of the certificates are being posted in stores. The central and southern parts of Florida stopped harvesting tomatoes about six weeks ago.

"The typical shelf life of a tomato is two to four weeks, so it's reasonable to assume that if these tomatoes did originate from Florida, they were harvested prior to May 1 and will no longer be in circulation or in retail stores," Acheson added.

The FDA has stepped up sampling of tomatoes coming across the border from all parts of Mexico. "Without a specific region to focus on yet, we have increased sampling from all parts of Mexico," Acheson said. "There is one part of Mexico -- Baja -- where they were into harvesting at the time of concern and they are on the list of exclusions," meaning they are considered safe.

The list of exclusions -- or safe regions -- now includes 37 states, Puerto Rico, parts of Florida and six countries -- Belgium, Canada, the Dominican Republic, Guatemala, Israel, and the Netherlands, according to the FDA Web site.

The message to consumers continues to be that grape tomatoes, cherry tomatoes and tomatoes still attached to the vine have not been linked to the outbreak and are safe to eat.

Roma, plum and red round tomatoes and products containing these tomatoes -- the ones implicated in the illnesses -- are safe to consume only if they are from areas that have been excluded from the ongoing investigation.

More information

Visit the U.S. Centers for Disease Control and Prevention for more on the salmonella outbreak.