MADRID , Spain ( CNN ) -- Letters containing white powder were mailed to 16 U.S. embassies across Europe , according to the State Department on Wednesday . Emergency services surround U.S. Embassy in Madrid , Spain . The substance has proved to be harmless in 15 locations with results still pending for the final embassy in The Hague , Netherlands , Deputy Spokesman Robert Wood said . He said the list of U.S. embassies where the powder was received includes Berlin , Germany ; Bern , Switzerland ; Brussels , Belgium ; Bucharest , Romania ; Copenhagen , Denmark ; Dublin , Ireland ; Luxembourg ; Madrid , Spain ; Oslo , Norway ; Paris , France ; Reykjavik , Iceland ; Riga , Latvia ; Rome , Italy ; Stockholm , Sweden ; Tallinn , Estonia ; and The Hague . An envelope , containing what testing revealed to be white flour , prompted the U.S. Embassy in Madrid to close the staff entrance on Wednesday , a U.S. embassy official told CNN . Mail at the U.S. Embassy in Madrid is received near the front entrance and was closed amid security concerns , he said , though most employees remained at work . `` We know where the package is coming from , '' the official said , but declined to elaborate . The official spoke on coindition on anonymity because he is not authorized to speak to the media . He described the incident as `` very serious '' but went on to say there is probably `` more commotion '' outside the embassy , where local media reported police and ambulances had gathered . A similar letter was mailed to the U.S. Embassy in Copenhagen on Tuesday , embassy spokeswoman Melissa Ford said Wednesday . She said the contents of that envelope `` still are n't in , '' contrary to Wood 's statement . `` It usually takes more than 24 hours ( for the cultures ) , so perhaps we 'll know later today or early tomorrow , '' Ford told CNN . CNN 's Elise Labott and Madrid Bureau Chief Al Goodman contributed to this report .
Sixteen U.S. embassies in Europe receive mail containing white powder . The FBI is investigating the incident as a possible act of terrorism. U.S. embassy official in Madrid : `` We know where the package is coming from ''