just read this in a Security email ...
The FBI has dropped a case against an alleged sex offender rather than reveal information about the methods used to obtain evidence against him. Authorities arrested a Washington state middle school teacher for allegedly accessing a website through the Tor network known to host child abuse material (pornography). Federal investigators were surreptitiously operating the site at the time, using a tool that revealed Tor users' IP addresses. A judge ordered the FBI to disclose the source code for the tool it used to identify the suspect but instead, the FBI has dropped its appeal.