The 46-year-old suffered a cardiac arrest while being restrained by Minneapolis police, the report found.
It listed Mr Floyd’s cause of death as “cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression”.
Meanwhile, President Donald Trump vowed to use the military to end the unrest.
A video showing a white police officer continuing to kneel on Mr Floyd’s neck even after he pleaded he could not breathe has reignited deep-seated anger over police killings of black Americans.
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It has led to six consecutive days of protests around the United States and a level of civil unrest not seen in decades.
The officer, Derek Chauvin, has been charged with third-degree murder and manslaughter and will appear in court next week. Three other police officers have been fired.