Health officials said all indications pointed to a single geographic region as the source of the outbreak, which has sickened 228 people in 23 states.

David Acheson, associate commissioner for foods at the U.S. Food and Drug Administration (FDA) said on a conference call that nine people who became ill with Salmonella had eaten at two different outlets of the same restaurant chain. He declined to name the chain or the location of the restaurants. "That represents a small cluster within this outbreak," he said.

The outbreak has been disastrous for the U.S. tomato industry, which produced $1.28 billion of the fruit last year.

The FDA has linked the outbreak to raw round, plum and Roma tomatoes, and has issued a list of states and countries whose tomatoes are not associated with the outbreak.

California, Georgia, New York, Canada, Guatemala and the Dominican Republic are among tomato producers on the safe list. The FDA also has cleared counties in North Florida, whose tomatoes were not widely available at the time of the outbreak.

The FDA said investigators were focusing on South and Central Florida and Mexico because they were the biggest producers at the time.

The Mexican government says its tomatoes are being unjustly targeted and notes that the uncommon Salmonella Saintpaul bacteria identified in the outbreak has never been found in Mexico.

The agriculture ministry said in a statement on Friday it would send a delegation to the United States next week to press officials there to declare Mexican tomatoes as safe to eat.

"The borders have not been closed to Mexican tomato imports but the alert put out by the U.S. government had provoked a decline in consumption," the ministry said.

Some shipments from Mexico have been stopped at the border by worried importers, raising concern among some analysts that Mexico could be hit by a glut of rejected produce.