Analysts predicted more than a dozen crashes could occur - or roughly one every two or three years.
Despite this, the Federal Aviation Administration (FAA) did not ground the aircraft until after a second crash five months later.
FAA chief Steve Dickson said the agency had made a mistake in its oversight.
The FAA risk assessment was revealed during a congressional hearing on Wednesday. Lawmakers are investigating Boeing following fatal 737 Max crashes in Indonesia and Ethiopia, which together killed 346 people.
