What "free market" monopolies have existed in the USA?

by The_Ripper42

My history teacher said that there's only one monopoly in the history of the USA that was a result of "free market" and not somehow affected by the powers of the government. He also said that the company was not just in the US.

ParkSungJun

A monopoly and a free market are mutually exclusive so I am not sure what this statement means. Can you clarify the context?