It seems like such a big clumsy plot to be planting evidence like those fragments.
But what if those fragments didn't match the other planted evidence? Those fragments might have been found in the limo but there is no provenance for it or practically any of the evidence in this case. Why did they "clean up" the limo by removing and destroying the windshield with the bullet hole in it? Wouldn't it be insane to tamper with crucial evidence found in the limo? And if they were willing to scrub the limo, then why wouldn't they swap out or manufacture bullet fragments previously shot by the planted rifle? The CIA and FBI weren't amateurs at this stuff.