Fired CIA guys hired themselves out to the Mafia to do the hit, and the government just covered up after the fact because it made many powerful agencies look bad in various ways - the CIA and ONI couldn't control their own, the FBI dropped the ball (seems like that happens a lot), Johnson probably knew something and just ignored or downplayed the information he got (or helped plan - God knows he hated the Kennedys).
On Bobby, Sirhan clearly acted alone in the actual execution of the crime, though his motivations could easily have been to help relatives or friends and Mafia involvement is definitely possible.
ἡ δὲ κἀκ τριῶν τρυπημάτων ἐργαζομένη ἐνεκάλει τῇ φύσει, δυσφορουμένη, ὅτι δὴ μὴ καὶ τοὺς τιτθοὺς αὐτῇ εὐρύτερον ἢ νῦν εἰσι τρυπώη, ὅπως καὶ ἄλλην ἐνταῦθα μίξιν ἐπιτεχνᾶσθαι δυνατὴ εἴη. – Procopius
Ummaka qinnassa nīk!
*MySmiley*