(Perhaps initially, as it went from mostly clean to compromised, there was bystander effect plus other effects)
An individual would be suicidal to join the LA Sheriff's force and try to solve it from within.
US military force needs to take control of the department, restructure the command, flush the compromised, and monitor as control is transferred back. That is what we would do if the force were compromised by a foreign force, and the same logic applies.