Elliot Rodger, 22, murdered six people and injured 13 others when he went on a gun and knife rampage near Santa Barbara in May after detailing his plans in a 141-page "manifesto".
Rodger's father - Hollywood director Peter Rodger, who worked on The Hunger Games - has spoken for the first time since the tragedy in an interview on US TV.
He said: "Every night I go to sleep, I wake up and I think of those young men and young women that have died, and were injured and were terrorised. And my son did that."
Mr Rodger said he had been living in a "reverse nightmare" since the massacre.
"When you go to sleep normally, you have a nightmare and you wake up and everything's OK," he said.
"Now I go to sleep and I might have a nice dream and then I wake up and slowly the truth of what happened dawns on me, and that is that my son is a mass murderer."
Rodger uploaded a YouTube video detailing his killing plans on the day of the shootings, which he labelled "the day of retribution".
Following his rampage, he was found dead inside his crashed vehicle with a gunshot wound to his head.