Envelopes with white powder have been sent recently to more than 40 US governors offices and 15 American embassies in Europe, but authorities have found no dangerous substances.
State Department deputy spokesman Robert Wood said the envelopes were mailed to embassies in Berlin, Berne, Brussels, Bucharest, Copenhagen, Dublin, Luxembourg, Madrid, Oslo, Paris, Riga, Rome, Stockholm, Tallinn, and The Hague.
He said all initial tests have proved negative, adding that results from The Hague were still to come in.
The FBI said in a statement meanwhile that the offices of more than 40 governors across the country received similar envelopes with a suspicious white powder but the contents tested negative.
Spanish security services cordoned off the US embassy in central Madrid on Wednesday after one of the suspicious envelopes sparked what turned out to be a false security alert.