If the he's mixed up in his Islamist brother's plots, the FBI should be 'hounding' him to get to the bottom of whatever was behind the failed OU attack, but really: ". . .I'll bury you," at the end of a exaspirated tirade won't pass muster as a threat under any of the versions of true-threat doctrine set forth by the various courts of appeals.
Probably not. It sounds like the FBI is using that as leverage to try to get something from him - or possibly from someone else.