Three weeks after the Food and Drug Administration first warned consumers to avoid certain types of tomatoes for fear they would carry an uncommon type of Salmonella, known as Saintpaul, the number of people getting infected is rising with each day passing by, making U.S. health authorities question whether tomatoes are the real or only cause for the outbreak.

On Monday afternoon, there were 851 cases reported in 36 states and Washington D.C., some of whom fell ill as recently as June 20, Robert Tauxe, deputy director of the Centers for Disease Control and Prevention’s division of foodborne diseases said, according to the USA Today.

As the number of infections is still rising, the FDA continues to be concerned with the fact that people are still exposed to this uncommon strain of Salmonella. According to Dr. David Acheson, associate commissioner at the FDA, tomatoes may not be the only ones responsible for the outbreak. Investigators have collected about 1,700 tomato samples and all of these have been negative.

Therefore, other ingredients may have contributed as well, and tests will continue until a source is identified.

“Produce investigators are very difficult, because a lot of times, vegetables are eaten together. We continue to keep an open mind about the possible source of this outbreak, as does FDA,” Dr. Patricia Griffin, chief of the Enteric Diseases Epidemiology Branch at the Centers for Disease Control and Prevention in Atlanta, Georgia, said, as quoted by CNN.

The investigators are currently analyzing water and other environmental samples in order to establish what the real cause of the Salmonella outbreak may be. However, there is no guarantee that the source will ever be found.

Thousands shoppers and growers who have seen million of tomatoes taken off grocery shelves in the last month received the health authorities’ news with anger.

Many businesses have been hurt by the ongoing salmonella outbreak in the US. Moreover, if infections are not linked to tomatoes, the government food safety system will face criticism now more than ever.

The same system has been attacked for failing to prevent illnesses and deaths resulting from contamination of a string or products, including peanut butter and bagged spinach, in recent years.