The killers were two crewmembers from the Enterprise. They tried to frame Kirk for the Chancellor's death to stop the Federation and the Klingons from making peace. The ship that fired on the Chancellor's ship was actually underneath the Enterprise and was a cloaked Klingon ship. Valaris, Chang, Colonel West, Cartwright and the Romulan Ambassador were all in on it. At the end of the movie, the Entrprise is retired and the cast literally sign off.
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