Well, you really need to watch the show more often, bud... you're asking about MAJOR plot lines here. Cohen is an ATF agent that was stalking the Doc. She used to have a relationship with him when they lived in Chicago. She came back to Charming to escape him because he was turning violent and took out a restraining order. He ignores it, starts going after SAMCRO and does some vile shit to Jax because he knows he's with the doc. Jax escalates the situation, first by shooting out his radiator, then beating the shit out of Cohen in a barber shop (which the barber claims Cohen started lolz). During this, it comes out that he's "on vacation" and the department thinks he's in Arizona or something. The female ATF agent and Hale kick Cohen out of town and supposedly drive him to the airport with Jax looking on. Cohen comes back that night, breaks into the doc's place and tries to rape her... one of the creepier scenes in television, imho. She tricks him into a feeling of calmness, then grabs his gun and shoots him in the stomach. She calls Jax over, and as he's cursing at her, "once a biker slut, always a biker slut," Jax gets all and shoots him in the head. Then they make sweet love on the bed as his body cools on the floor scant feet away. Jax later burns the body and initially tries to burn his father's manuscript on top of the body. He then thinks better of it and pulls it out, but it's halfway burned. This is the copy that Gemma is now reading. The black officer is Trammell, who has ties to SOA as "help."