The information comes after relatives have pressed for an independent autopsy of Floyd's body, reports the news agency AP.
The medical term is "mechanical asphyxia", which occurs when the respiratory tract is blocked, Reuters reports.
Video sequences from the scene where Floyd died show how he lies handcuffed on his stomach on the ground next to a police car, while a police officer pushes his knee to his neck. George Floyd repeatedly appeals: "Please, I can't breathe".
When the cop put pressure on Floyd's neck and back, his blood flow to his brain was cut off and the weight over his back made it difficult to breathe, said family lawyer Ben Crump.
The doctor who performed the autopsy has also autopsy Eric Garner - a 43-year-old man who died of asphyxiation as a result of police intervention in 2014, which sparked widespread protests in the United States.
An earlier autopsy has not shown suffocation as a cause of death, but argued that Floyd had a heart condition, which should have contributed to his death during the intervention.