The AP photos taken of the assault on the Fallujah hospital reveal that some of the troops involved were using the AK-47 assault rifle favored by the Iraqi resistance instaed of the m-16/m-4 weapon issued to US troops. The M-16 fires a 5.56 mm bullet, while the AK-47 fires a 7.62mm bullet. It would be easy to differentiate casualties caused by US weapons if they stuck with the M-16. The switch indicates that they were perhaps preparing to blame insurgents for any civilian casualties.
Watch for more of this as the reports roll in. The AK-47 is differentiated by its curved magazine and distinctive gas tube sticking out of the handguard over top of the barrel.