WHO is making the threats?
George Soros' stench is all over this. Start there, and one of his many organizations.
I have looney left relatives who are certain these threats come from Trumpers, Christians, the KKK, etc., and they are adamant that there is no way that fraudsters, lefties, and those nice Islamist refugees could be behind it.
We don't know exactly yet from information that is available in the media, but the first guess would be Islamic terrorists. They have been active in the US lately, and they have hit Jewish targets in other countries many times before. Specifically, They hit a Jewish community center in Argentina a number of years ago with multiple deaths resulting.