Conversation so far:
What is Kenneth, what is the frequency?
Rather was attacked and punched from behind by a man who demanded to know "Kenneth, what is the frequency?.
What happened after the attack?
The assault remained unsolved for some time, and was referenced multiple times in popular culture.
Was the attack ever solved?
In 1997, a TV critic writing in the New York Daily News solved the mystery, publishing a photo of the alleged assailant, William Tager.
Was William punished for it?
William Tager, who received a 12 1/2 -to-25-year prison sentence for killing NBC stagehand Campbell Montgomery outside The Today Show studio in 1994.
Was the frequency ever made reviled?
" Tager claimed he thought television networks were beaming signals into his brain.

Question: What proof did he have?
Self-contained rewrite: What proof did Dan Rather have?