Maternal Grandfather

The father of my mom was German, he emigrated to the United States after leaving Europe for good in early 1950-s. My grandmother lost the contact with him for a long time after sending him the last photograph of herself with my mother as a little girl, I've seen a copy of it. She refused to talked about him, probably because of pain and caution. I know the story primarily from my aunt. He had another son in USA. I arrived here too late to meet either of them.