At least 13 deaths and 72 illnesses have been linked to a listeria outbreak from Colorado cantaloupes, health officials say, in the deadliest food outbreak in more than a decade.
Three other deaths may also be related to the tainted fruit, which are linked to a farm in Holly and have been recalled.
Bacteria were also found on equipment and produce at the farm's packing site.
The FDA is investigating how the contamination may have happened.
The Centers for Disease Control and Prevention (CDC) warns the number of incidents is likely to grow, since symptoms can take four weeks or more to appear.
"That long incubation period is a real problem," Dr Robert Tauxe of the CDC said.
"People who ate a contaminated food two weeks ago or even a week ago could still be falling sick weeks later."
Most healthy adults are unlikely to suffer ill effects from listeria, however the elderly and people with weakened immune systems are more vulnerable.
Correspondents say the median age of those affected is 78.
Since 31 July, incidents have been reported in 18 US states with fatalities confirmed in areas as disparate as Texas, New Mexico and Maryland.
Listeria bacteria can grow at room temperature, even refrigerator temperatures, unlike many other pathogens.
The CDC and US Food and Drug Administration (FDA) have advised that all potentially contaminated produce is thrown away immediately, and that any surfaces it may have touched are sanitised.
In 1988, 21 people died in a listeria outbreak from contaminated hot dogs, while in 1985, 52 deaths were linked to listeria contamination in Mexican-style soft cheese.
The CDC says around 800 listeria cases are reported in the US each year.
