Who does Douglass believe to be his biological father?

Frederick never knew his father, what we learn from his narrative is as follows.....

My father was a white man. He was admitted to be such by all I ever heard speak of my parentage. The opinion was also whispered that my master was my father; but of the correctness of this opinion, I know nothing; the means of knowing was withheld from me. My mother and I were separated when I was but an infant—before I knew her as my mother.