When it comes to federal agencies, who is scarier than the FBI (besides the IRS, CIA, DEA, INS and Secret Service). Looks like scammers are taking advantage of the authority of the FBI name and launching a Nigerian Scam using FBI Director Robert S. Mueller III.'s name.
Just a tip, if you get an email from Robert W. Mueller, it's probably a HOAX. He is a high ranking government official and probably doesn't have time to send YOU and email.