Jens Breivik, a retired diplomat, lives in a village in the Aude region in the south of France.
“I was reading about what happened online when I suddenly saw his name and photo,“ he told Norwegian newspaper Verdens Gang.
His son, Anders Behring Breivik, is under arrest for detonating a bomb in central Oslo on Friday and then shooting a large number of people, many of them teenagers, on the island of Utoya.
Jens Breivik told the newspaper he divorced his son’s mother shortly after he was born and that he and his son have been estranged since 1995.
“We have never lived together although we were in touch when he was a child,” said Mr Breivik.
“When he was young he was just an ordinary boy, although quite reserved. He wasn’t interested in politics back then.”
Mr Breivik remarried and worked as a diplomat in London and Paris. He claims to have asked for custody but this was denied.