The Minnesota Democrat called suggestions that he is anti-Semitic – based on his involvement with Louis Farrakhan and the Nation of Islam while he was in college – "smears."
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"We are working with law enforcement officials to determine if it is connected to similar threats against Jewish institutions across the country," ADL CEO Jonathan Greenblatt said in a statement.
The celebrated Jewish-American author said academic boycotts against the Jewish state were the result of Benjamin Netanyahu's policy of excluding the country from the Western world.
The 6-foot-high gate, stolen in November 2014, was found last December in a parking lot in Norway following an anonymous tip.
The free application, which asks incoming freshmen questions about their Jewish identity, was launched at the teen group's huge annual convention.