All military police in the British Army (whose mission encompasses what our Army's CID does) can not become MPs until they've reached at least the rank of Lance Corporal in another MOS, and many come from the combat arms...IMHO, not a bad system; however, their MPs don't have all the battlefield missions ours do, so we have a legitimate requirement for more privates. To become a CID agent in the US Army, IIRC, one must be at least an E4, promotable to Sergeant. While most of those do come from the ranks of line MPs, that's not a hard and fast rule, and I've known several who came from some pretty divergent MOSs.
The thing about Tillman's death is, CID would not even look into the matter if there was no suspicion of criminality. In the event of fratricide, I would suspect the first O6 in the chain of command would have appointed an investigating officer to conduct a commander's inquiry under AR15-6, and the findings would have stayed within the chain.
Fratricide is, sadly, just another one of the grim realities of war, but I think in this case, the Army is largely wrestling with a public relations nightmare of its own making.
Fraticide may be a reality of war, but with these young men in uniform, highly trained volunteers, Rangers, I suspect very, very rare. Except for one story of questionable reliability, there is no indication Tillman was a victim of such.
No, officials reporting these matters to the press is a major mistake and if it continues, over a period of time, will result in the serious reduction of the military’s effectiveness. Contrary to widespread opinion, the public has no “right” to be fed all these stories.