A co-worker of the Florida man who died of anthrax last week has tested positive for the extremely rare disease.
Health officials last week described the case of the man who died as an isolated one that was not a biological attack. But the FBI said today it was urgently investigating the source of the infections.
"Every step is being taken to quickly identify the bacteria's source and determine how the two individuals were infected," the FBI said in a statement.
"A gentleman who worked with the other patient had a cotton swab taken of his nose and that tested positive but he does not have the illness", said Mr Frank Penela, a spokesman for the Florida Department of Health.
News of the second case came as the United States was on high alert for possible attacks by the al Quaeda network associated with Osama bin Laden.
US security services, fearing a retaliatory terrorist attack in America, are on high alert following strikes on Afghanistan last night.