A a law enforcement official describes Europe's isecurity environment as "difficult."
Europe, US Officials Warn of Severe Terror Threat
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German police officers patrol the airport terminal in Frankfurt, Germany, Jan. 19, 2015.
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A Belgian soldier guards an entrance to the European Council headquarters in central Brussels, Belgium, Jan. 19, 2015.
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Belgian soldiers patrol the EU headquarters in Brussels, Belgium, Jan. 19, 2015.
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In the midst of heightened security, soldiers patrol the EU headquarters in Brussels, Belgium, Jan. 19, 2015.