It ain’t the ammo, it’s the gun. I know I’m a little late to the discussion, but any decent AR platform is designed for 5.56 ammo and will shoot .223 no problem. The issue is with guns designed for .223, like a bolt action varmint rifle, can’t handle the pressures of a 5.56. A .223 or 5.56 will feed just fine in either gun. The bullet (say a 55 gr.) is identical, the casing of a 5.56 is a little heavier and are loaded a little hotter.
That's what we here eventually found out with the links at #112 and #116, despite some here getting their diapers all in a bunch over someone claiming that there was a lot of 5.56 brass casings all over the floor while the state police list the weapon used as a Bushmaster 223 caliber model XM15-E2S rifle.
Since 5.56 and 223 ammunition are apparently interchangeable in that model, it wouldn't be surprising to find a lot of one or the other or both.
Either way, whether 5.56 or 223 or a mix of each, the brass casings would point to the Bushmaster.