Here is a story from the Talmud with relevance to current events:
It once happened that Tuvia sinned and Zigud alone came and testified against him before Rav Papa. Whereupon, Rav Papa had the witness, Zigud, punished. Zigud exclaimed: Tuvia sinned and you punish me, the witness! Rav Papa said to him: It is written in the Torah: "A single witness shall not rise up against a man." [Deuteronomy 19:15]. When you testified against him alone, all you did was to bring him into disrepute. You knew you were the only witness and you knew I could not condemn him on the basis of a single witness, so I am having YOU punished. [Tractate Pesachim, 113b]
My question is: is it the judge's job to order more investigations, or simply to rule based on what is brought before him?