When you start stacking up evidence that by itself is inconclusive, you find that the overall probability starts to get very tight.

For instance:

    suspect was in this neighborhood at this time
    suspect was wearing a blue jacket
    suspect had short blonde hair
    suspect was an acquaintance of the victim

Each of those by itself means next to nothing; you can't go around arresting anybody with blonde hair because there are millions of people with hair like that. But if you find somebody who fits all four, they are almost certainly your guy. The chance of there being two acquaintances of the victim with blonde hair and a blue jacket in the neighborhood at the time of the murder is much smaller than the chance of a false positive on any one of those individual conditions.

With enough circumstantial evidence, you can blow right past "reasonable doubt".