How may police officers be held criminally liable?
If a police officer exceeds the bounds of their authority in any one or more of a number of ways, then they are liable for criminal charges (may be criminally liable) if/when charges are proven. There are several categories for criminal liability as well as some non-criminal categories.
The criminal categories include assault, tampering with evidence or vehicular tickets, misusing department funds, and drug or murder crimes. "Exceeding authority" also includes exerting excessive or undo force against suspects or defendants in any part or parts of the police process from stops to detention.
This aspect of criminal liability has come under greater scrutiny in recent decades because of the prevalence of witnesses to such actions carrying mobile phones (cell phones) and using the camera and/or video features of these phones to record police brutality.