If you make a plea bargain with the police department to reduce charges, will the District Attorney back the police department's promise?

Not necessarily. Police and other law enforcement agents (such as D.E.A. or F.B.I agents) do not have the ability to make binding plea agreements or sentence concessions. Only a prosecutor can make a binding promise that you will not be charged with a crime or that you will receive a lesser sentence if you cooperate, confess or agree to plead to lesser charges. Police can tell you they will make your cooperation known to the prosecutor. They can tell you they will recommend to the prosecutor that you receive a more lenient sentence or that all or some charges not be filed against you. But they cannot make promises that are binding on a judge, and while prosecutors will surely consider their recommendations, they are not bound by them either. If the police have made you such a promise, I suggest you seek legal counsel in your area to confirm the promise with the prosecutor before you rely on it -- and to advise you as to whether the promise is in your best interests, even if it is confirmed by the prosecutor.