如果是通过VPN或者Proxy Server的话,FBI作为Law enforcement agency, 完全可以越过隐私保护法,通过法庭强制令从VPN/proxy server provider/Internet service provider得到相关数据库(如logs of user activities including network traffic)、历史使用情况和行为分析,利用黑客暴露的miss configurations and leaks, 追踪到original IP address.
After all, there is no 100% guaranteed anonymity. 互联网有记忆,江湖飘有痕迹 :)