I've never heard of any P-51 Mustang being shot down over the US during World War 2. I do know there were training accidents, but I've not heard of any involving friendly fire, but I don't think it impossible. Gunnery training did incorporate some specially modified, manned aircraft such as the Bell RP-63 Kingcobra Pinball which had extra armour, special sensors, and a lighting system in the nose to record successful hits by frangible, coloured bullets, but there's no record of a P-51 Mustang being used in this way.You may want to pose the question at the forums listed below.
If it's any help, I have heard of a WWII era aircraft( P-51 or P-38) being used by drug traffickers that was shot down by the Coast Guard a few years back,just a rumor as far as I know, maybe that's what you're thinking of.
AS FARE AS I KNOW THERE WAS NOT A P-51 MUSTANG SHOT DOWN OVER US SOIL. BUT YOU MAY FIND THE INFO YOU ARE LOOKING FOR AT THIS WEB SIT http://www.globalaircraft.org/ OR AT ONE OF IS HUNDREDS OF OTHER WEBSITES LINKS. ALSO YOU CAN FINED ALL YOUR AIR PLANE INTERESTS THERE ALSO.