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.
Argentina had the goods on those who blew up the Jewish community center in Argentina {Iranian agents) and was about to bring tthem to trial
when Obama warned them off
saying that that would hurt his upcoming “deal” with Iran.