On Tuesday, prosecutors dropped charges against another man, Paul Kevin Curtis of Corinth, Mississippi, who was arrested April 17 and accused of sending a threat to the president after letters containing a suspicious powder triggered security scares around Washington.

CNN: Mississippi man arrested in ricin case

This latest patient is an NBC "Nightly News" employee in New York who received mail containing a suspicious white powder on the third floor of Rockefeller Center in Manhattan.

CNN: Bush: U.S. still in danger, but strong