alt Hacker NewsLaw and Order lied to us. Courts run on circumstantial evidence. A lot of cases turn on it.
For criminal law, prosecutors have to prove beyond reasonable doubt. If they have a gun, search history for "how to kill someone and get away with it", blood in their car, and someone that's been missing for an extended period of time. They can and have gotten convictions based on that. Hans Reiser was convicted under pretty much those circumstances.
Your example evidence set is actually quite robust by US justice system standards. There are countless cases of innocent people getting sentenced to death for e.g. being the one to report the murder, often in the face of strong circumstantial counter evidence.