Attorney General Eric Holder urged European countries Tuesday to become more aggressive in countering the threat of Westerners who receive training from terrorists in Syria.
"This is a global crisis in need of a global solution," Holder said. "The Syrian conflict has turned that region into a cradle of violent extremism. But the world cannot simply sit back and let it become a training ground from which our nationals can return and launch attacks."
He warned that Western democracies cannot be passive.
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"If we wait for our nations' citizens to travel to Syria, to become radicalized, and to return home, it may be too late to adequately protect our national security," Holder told a group of diplomats and security officials in Oslo, Norway.
Recently tightened air passenger screening requirements for flights to the United States are the result of fears that terrorist groups in Syria may seek to exploit the ability of Europeans to travel easily to the U.S. without needing visas.
Holder cited U.S. intelligence estimates that nearly 23,000 violent extremists are operating in Syria.
"Among these are over 7,000 foreign fighters," which he said included "dozens of Americans."
Holder urged other European countries to follow the model of France and Norway in passing laws that criminalize activities undertaken in preparation for terrorist acts. Another example, he said, is the U.S. law, strengthened after the 9/11 attacks, against providing material support to terrorist organizations.
Norway's intelligence agency estimates that as many as 50 of its citizens have traveled to Syria. Undercover operations can help disrupt terror plans in their early stages, Holder said, noting the FBI's use of sting operations to identify people in the US with extremist leanings. But he cautioned that such investigations must not stray into attempts at entrapment.
Holder also urged greater sharing of information among European countries and the U.S. about who has traveled to and from Syria as well as more outreach to immigrant communities to counter radicalization.