Conversation so far:
What is Kenneth, what is the frequency?
Dan Rather was attacked and punched from behind by a man who demanded to know Kenneth, what is the frequency? while a second assailant chased and beat him.
What happened after the Kenneth attack on Dan Rather?
Until the crime was resolved years later, Dan Rather's description of the bizarre crime led some to doubt the veracity of his account.
Was the Kenneth attack on Dan Rather ever solved?
William Tager's identity as the man who attacked Dan Rather was established in the course of an investigation.
Was William Tager punished for attacking Dan Rather?
William Tager received a 12½-to-25-year prison sentence for killing NBC stagehand Campbell Montgomery outside The Today Show studio in 1994.

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