I literally just talked to an attorney about this very thing. He is my counsel for any gun-related incident. This is the advice he gave me...and you're getting it for free!
If you are holding the BG at gunpoint, tell the 911 operator that the assailant is down and the victim has him at gunpoint. Give a description of the victim (yourself - height, weight, clothes, gun type).
If BG is incapacitated, move away from BG but DON'T LEAVE SCENE! Holster weapon. Tell the 911 operator that assailant is down and may need medical assistance. Also tell 911 operator that all firearms are put away. Again, describe the victim (yourself).
When police arrive, COMPLY WITH ALL COMMANDS. Even if an officer tells you to lay on the ground and cuffs you. Surrender your weapon. Only answer the most basic questions, and you may do those by handing them or having them retrieve your driver's license. Name, Address, phone number.
DO NOT ANSWER ANY QUESTIONS ABOUT THE ENCOUNTER, THE TIME BEFORE OR AFTER IT. Not why you were there, what happened, what the BG said, how many shots you fired, NOTHING. No matter how right you think you are and how innocent you think the question is, YOU ARE WRONG. If the officer asks you what color jacket the BG was wearing, and you LOOK at the BG and see a BLUE jacket, refuse to answer.
The proper, respectful, POLITE answer to all questions is, "I will be more than happy to cooperate with your investigation, but I do not feel I should answer any questions before speaking to my counsel. Please contact him. His name is Mr. _____________ and his phone number is _____________. Here is his card." If a different officer, or even the same officer asks you another question, you state even more respectfully and more properly, "As I told the other officer, (or "As I told you before") I will be more than happy to cooperate with your investigation, but I do not feel I should answer any questions before speaking to my counsel. Please contact him. His name is Mr. _____________ and his phone number is _____________. Here is his card."
Never seem rattled or annoyed by their questions. They may be trying to rattle you, but they may also be confused and lost on their first shooting case. The duplicate questioning may be innocent. Just remain cooperative, friendly, respectful and polite so those are the only words EVERY police officer can use with the Jury, Grand Jury, Attorney General, and all the others that are going to decide far ahead of time whether or not to charge you with a crime.
As soon as they allow you, call your attorney and give him a quick, concise breakdown of the shooting and where you are being held. Do not let the police hear ANY of your conversation with your attorney and assume the line is bugged, so no admissions of guilt like, "I don't think he actually drew his gun" or other self-incriminations like that. Wait until you and your attorney are together and he tells you the two of you are completely secure.
Do EVERYTHING your attorney tells you to do and NOTHING he doesn't specifically tell you to do. Do not get creative and think for yourself - you will screw up his defense of you! If you have an idea for your defense, call your attorney. By state law, you are ALWAYS allowed calls to your attorney when regarding your defense.
The end.
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This is the end of the attorney advice, but I have a related story that will always cement this advice in my head as the way to do it and the things that can go wrong with police involvement at the scene.
My car was down at one point with a major repair that had it in the shop for a couple of weeks, so I was borrowing my father-in-law's Honda hatchback. A contractor with all of his attention on his cell phone and none on the road changed lanes and ran me off the road and head on into a parked car. A secretary on her way to lunch and my Wife in the passenger seat both witnessed the entire event. I called police to have a proper report for the insurance company.
The female officer that arrived didn't believe that I had been run off the road by a contractor's oversized white pickup because there were no signs on my vehicle of where it struck me. I explained I kept moving over to get out of his way until I couldn't move over any more and BAM!! Into the parked car - just ask my witnesses. Then the officer said, "You didn't even try to stop...there are no skid marks." By this point my Wife is getting madder and madder at the insolence of this officer. She gets out of our wrecked vehicle, stomps over between our hatchback and the officer's service mini-pickup, points to the ground and says, "You mean the skid marks you parked right on top of???" She scowled at the officer, frozen in the pointing stance...the officer spins on her heals to go talk to the other witness.
When the officer came back, I had to argue with her for 30 minutes before she finally said, "Well, I don't see it the way you say it happened, but I'll write it up that way." When I got the report a couple of weeks later, even with two unimpeachable witnesses, it was full of "supposedly's" and "allegedly's" to make it sound like everything out of my mouth was a G___ D___ Lie. The Insurance Co. paid and I got on with life, but when it happened, all I could think about was the horror of telling my FIL that I had totalled the car he let me borrow. Then I was basically psychologically tortured by this officer who chose to ignore the evidence, the witnesses and the fact that I had no criminal record, did no drugs or alcohol and had no reason to lie... as did all of the witnesses.
This incident was one officer with a minor traffic accident and had nothing to do with my LIFE. Imagine if this travesty had been a dozen investigators, Lieutenants, Captains, a dead body against me, my gun and my CCW. Doesn't it make you shudder???