As the deadliest recorded outbreak of E. coli illness continued to expand in Europe on Friday, U.S. officials sought to reassure the public that the food supply in this country is safe and they do not think the illness will spread here.

“This outbreak has not affected the U.S.,” said David Elder, director of regional programs at the Food and Drug Administration. “Produce remains safe, and there is no reason for Americans to alter where they shop, what they buy or where they eat.”

The outbreak that has emerged in Europe has been linked to a particularly virulent form of E. coli 0104 that has killed 19 people, sickened more than 2,000 and caused at least 550 to develop a life-threatening kidney complication that is not treatable.

Four people in the United States who had visited Hamburg are believed to have been sickened in the outbreak. Three of the four remain hospitalized in the United States with the kidney complication known as hemolytic uremic syndrome. The fourth person is being closely monitored, said Chris Braden, director of foodborne, waterborne and environmental diseases at the Centers for Disease Control and Prevention.

In addition, two U.S. military service members based in Germany are also believed to have fallen ill, Braden said.

In a conference call with reporters Friday, Braden, Elder and other federal officials said they are carefully checking Spanish and German produce imported into the United States. Although as much as 50 percent of the fresh fruits and vegetables consumed in the United States comes from other countries, less than 1 percent comes from Spain or Germany, Elder said.

European investigators have been interviewing victims of the outbreak and analyzing what they ate in an effort to identify the cause of the illness and stop its spread. Germany’s disease control agency said Friday that victims of the outbreak ate more salad than people who have not fallen ill. Lettuce, tomatoes and cucumbers are among the suspect foods. Employees at one Frankfurt business who dined at the company’s cafeteria salad bar were seven times more likely to develop bloody diarrhea — a symptom of E. coli infection — than those who did not eat salad, according to German health officials.

There are about 700 varieties of E. coli, most of them harmless to humans. But a small number produce a toxin known as Shiga that can cause serious illness or death in people.

The bacteria live in the intestines of cows, pigs and other ruminants. Although it is not clear how E. coli transfers from cattle to produce, scientists think it spreads through contact with manure or tainted irrigation water.

In the United States, food safety efforts have been focused almost entirely on E.coli 0157:H7, the strain responsible for a series of high-profile outbreaks, starting with tainted hamburgers sold in 1993 by Jack in the Box that killed four children and sickened hundreds. But other strains — just as dangerous to humans as 0157 — have been emerging in food.