Four more people have died from tainted romaine lettuce, federal health officials said Friday, bringing the total to five deaths related to a virulent strain of E. coli whose source has still not been located.

In addition, officials at the Centers for Disease Control and Prevention updated the case count: 197 people from 35 states were sickened.

Food and Drug Administration officials said, however, that romaine now for sale on grocery shelves is safe to eat. The growing season in the Yuma, Ariz. region, which produced the contaminated lettuce, ended April 16. According to an FDA blog post, “Any contaminated product from the Yuma growing region has already worked its way through the food supply and is no longer available for consumption. So any immediate risk is gone.”

The F.D.A. said its investigators were still working to trace sources of the outbreak. While they have traced the toxic E. coli strain to the Yuma growing region, they are still looking for the precise source — whether it originated in the water supply, harvesting equipment, a processing plant in the area or somewhere else.