Pre-Arkham Origins Incident
At some point, the Gotham City Police Department, in particular the SWAT division, had become privy to its corruption, requiring a visit from Internal Affairs. The SWAT unit led by John DeMarco, himself already serving time for drug use, attempted to keep quiet from Internal Affairs and ensure they can't find anything pointing to their corruption. However, one of the SWAT officers, Nate Ramo, panicked and offered to make a plea deal with Internal Affairs. John DeMarco, having found this out, made measures to try and get rid of him permanently.
Arkham Origins Incident
DeMarco eventually made his move during the events of the Christmas Eve incident. He parked his sedan near the GCPD precinct at Burnley, and upon seeing Nate Ramo (who was presumably waiting for an IA officer to come at a meet), proceeded to speed up and hit him, instantly killing him. However, due to the inclement weather, he also ended up losing control of his sedan and hitting another car before driving off, although not without leaving behind some of his car's paint job on the wall, with his car carrying a distinct paint job. He then went over to Nate Ramo's house to see if he had any evidence at his apartment, and verified that he didn't. One of the GCPD cops stationed at the precinct, Officer Meyer, witnessed the hit and run, and notified dispatch of the event and that the person killed had been one of their own. As the other homicide units were out on calls, however, they needed to wait until one of them returned to the precinct before they could do an investigation.
Batman managed to learn about the hit and run from overhearing Officer Meyer's earlier request. Batman, upon arrival, realized immediately that the hit and run occurring just outside the GCPD meant the hit and run was definitely not an accident. He then ID'ed the victim as Nate Ramo, who was part of the SWAT division, and that his whole team was currently under investigation by Internal Affairs, although he wasn't sure why he was at the location. He then deduced that he had been hurled a considerable distance by the vehicle, and combined the falling splatter with the splatter of the initial impact deduced that the car that killed him had not only been travelling at high speed, but also lost control. He then found the paint job residue from the car responsible, and upon cross-referencing the paint job, the specs of the make and model, as well as the team members in Nato Ramo's SWAT team, deduced the murderer as being John DeMarco and decided to pay him a visit.
He then tracked John DeMarco and his SWAT team down to just down the street just as the latter mentioned he had gotten rid of Nate Ramo, to the protests of his fellow SWAT team members as they feared that would result in the IA investigating the murder and deducing their involvement, although he expressed confidence at their not finding anything and also making clear that he doesn't have anything incriminating at his apartment, and even IF he stashed some somewhere, his deceased status will not reveal where it was. Batman then took out the SWAT team, saving John DeMarco for last. He then interrogated John DeMarco for why he murdered Nate Ramo, and then informed him that his remaining years in jail for the drug charges were extended to life imprisonment for murdering a fellow officer, also expressing confidence that the jury will convict him with all the evidence he had collected. However, John DeMarco then implied he'll arrange for the evidence to "disappear", with Batman declaring he won't let that happen before knocking him out. He then tells Alfred to notify the GCPD that Nate Ramo's murderer has been apprehended, and that he should also send them a taped confession from John DeMarco, which is all but guaranteed to bring John DeMarco's SWAT team down.
Batman: A hit and run just outside GCPD? This was no accident.
Scan of victim
Batman: Victim is Nate Ramo, a member of the SWAT division. His whole team is currently under investigation by internal affairs. I wonder what he was doing here...
Scan of impact zone
Batman: Looks like he was thrown a considerable distance by the vehicle. I can use the splatter distribution to determine the point of initial impact.
Scan of initial impact zone
Batman: This is where Ramo was killed. DNA evidence indicates the car was travelling at a high rate of speed. But the angle of splatter shows the driver lost control and the car was weaving. I should review the crime scene - see if anything else was hit.
Reviewing the evidence collected so far
Scan of impact point
Batman: Chemical analysis indicates a specific brand of paint. That narrows down the make and model of the car. And cross-referencing these specs with a list of his team members returns a single name: John DeMarco. Time to pay him a visit.
Finding John DeMarco
John DeMarco: We're good fellas. He never made it inside.
SWAT: We're good?! You took him out in front of the freakin GCPD! They're gonna be all over this. And then they're gonna be all over us!
John DeMarco: Relax. With Ramo out of the picture they get nothing. This time next week, we'll all be back at work.
SWAT: What if he's got tapes or something? Some kinda written confession? This is bad.
John DeMarco: I searched his place after I took him out. It was clean.
SWAT: He could have stashed stuff somewhere.
John DeMarco: And if he did? It's not like he can tell anyone where it is. We just gotta keep our heads down and our mouths shut.
SWAT: I hope you're right.
John DeMarco: I *know* I'm right.
Batman arrives at the scene
SWAT: It's the vigilante!
SWAT members beaten
Batman puts John DeMarco into an interrogation lock
Batman: Why'd you do it, DeMarco? Why'd you kill Ramo?
John DeMarco: IAD didn't have enough to make the charges stick, but the kid got scared. Made a deal and decided to talk. Traitor.
Batman: You would have been out of jail in a few years on the drug charges. But now? Murder means life.
John DeMarco: We'll see about that.
Batman: With all the evidence I've collected, it won't take long for the jury to convict.
John DeMarco: Funny thing about evidence, it has a tendency to disappear.
Batman: Not on my watch.
Batman knocks John DeMarco out cold
Batman: Alfred, let GCPD know that Ramo's murderer has been captured. I'm also sending you a taped confession. We'll bring down the rest of the team with him.
Burnley Hit and Run Casefile Closed
Cop: Dispatch, do you copy?
GCPD Dispatch: Go ahead.
GCPD Dispatcher: Copy that. Will send you our first available.