The findings resulted from more than a thousand interviews of Rohingya men and women in refugee camps in neighbouring Bangladesh, where almost 700,000 Rohingya have fled after a military campaign last year in Myanmar's Rakhine State.
"The survey reveals that the recent violence in northern Rakhine State was extreme, large-scale, widespread, and seemingly geared toward both terrorising the population and driving out the Rohingya residents," according to the 20-page report.
"The scope and scale of the military's operations indicate they were well-planned and coordinated."
Survivors described in harrowing detail what they had witnessed, including soldiers killing infants and small children, the shooting of unarmed men, and victims buried alive or thrown into pits of mass graves.
They described widespread sexual assault by Myanmar's military of Rohingya women, often carried out in public.
One witness described four Rohingya girls who were abducted, tied up with ropes and raped for three days. They were left "half dead," he said, according to the report.
Human rights groups and Rohingya activists have put the death toll in the thousands from the crackdown, which was sparked by attacks by Rohingya insurgents on security forces in Rakhine State in August 2017.
The results of the US investigation were being released nearly a month after UN investigators issued their own report accusing Myanmar's military of acting with "genocidal intent" and calling for the country's commander-in-chief and five generals to be prosecuted under international law.
The military in Myanmar, where Buddhism is the main religion, has denied accusations of ethnic cleansing and says its actions were part of a fight against terrorism.
US Senior State Department officials said the objective of the investigation was not to determine genocide but to "document the facts" on the atrocities to guide US policy aimed at holding the perpetrators accountable.
A declaration of genocide by the US government, which has only gone as far as labelling the crackdown "ethnic cleansing," could have legal implications of committing Washington to stronger punitive measures against Myanmar.
"Reports of mutilation included the cutting and spreading of entrails, severed limbs or hands/feet, pulling out nails or burning beards and genitals to force a confession, or being burned alive," the report said.
Any stiffer measures against Myanmar authorities could be tempered though by US concerns about complicating relations between civilian leader Aung San Suu Kyi, a Nobel Peace Prize laureate, and the powerful military which might push Myanmar closer to China.