Quote from bigdavediode:
No Brandon, that's not what the Secret Service said. They did drop the investigation because they couldn't be sure who the death threat was addressed to, and the Republican crowds have been uncooperative.
http://afp.google.com/article/ALeqM5hQdKB_yggRkzx5eyQlueyGvsmt7g
Actually according to reports out of Sacramento none of the Secret Service agents present heard the threats, and when they asked members of the crowd if they had heard such threats only one person, the journalist who originally made the claim, report having heard it.