So a detective investigating a robbery should just ignore evidence they uncovered of a murder because why?
Let's go with your analogy....
A Detective should NOT be asked to investigate a crime that never happened, a made-up crime when no evidence even exists suggesting there was a crime.
That being said....
If this detective is asked to solve a robbery that just happened 'involving SUSPECT 1' - a robbery where there is no evidence a robbery even took place involving SUPECT 1, the detective should not be investigating the entire past history of the friend of Suspect 1 in an attempt to find ANY crime that has nothing to do with the supposed robbery.
And during the course of his investigation, the Detective should NOT completely ignore the undeniable evidence of crimes he finds, evidence that reveals his own Pals on the Police Force and in the local government broke the law....but that is exactly what Detective MUELLER has done.
Mueller continues to look for the non-existent evidence of the non-existent crime of Illegal Collusion by the President while refusing to acknowledge the real, tangible, undeniable evidence of crimes committed by hit fellow Deep State Obama Administration Co-Conspirators...while he continues to break the law himself.