Connecting dots.

CRIMSON HAT S04 E024

Luther Wainwright - Murdered - Kidnapped by RJ - Shot by an agent.

Where was he seen last and how did he get in that car?

He was last seen talking to Susan Darcy in his office.

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As the limousine is about to get away, she shoots for the car. When she opens the door, she finds Luther Wainwright on the backseat and discovers that she killed him.
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How does Luther get kidnapped from his own office?

Someone had to have been in the office at the time I think perhaps CBI Ron. Since (Luther, Ron & Bald guy) were getting on the elevator together - when PJ entered the precinct.

Then the 3 (Luther, Ron & Bald guy) rushed down together to see what the shots were about - When PJ "shot" Rigsby.

Perhaps the "meeting" they were having - which was interrupted by the gun shots - was continued after Susan had words with Luther about the incident.

In which then Luther was kidnapped..

Susan being one of many moles in FBI 

I don't like the idea of Ron being RJ purely because we barely know him. But what do we know of RJ? Nothing we don't know him at all.

Patrick only knows what RJ has left behind.

But RJ knows everything about Patrick. And where better to get to know him than work in the aisle next to them.