From the search warrant request:

Because the FBI cannot predict whether any particular formulation of a CIPAV to be used will cause a person(s) controlling the activating computer to activate a CIPAV, I request [...] to continue using additional CIPAVs [...] until a CIPAV has been activated.

Read "activate"="double-click on attachment". So much for the FBI exploting secret security holes that are otherwise unknown or actually paying OS vendors to install backdoors and security software vendors to not detect their spyware. Looks very much as if it's just a plain old trojan...