This is completely standard practice. It sounds like the initial summons listed only the operating under the influence charge but the criminal complaint now includes an additional criminal mischief crime. The state is not stuck with the charges that the officer wrote on the summons and may add or subtract offenses if they believe they're supported by the evidence. I should note that I am assuming you are wrong about the trial starting already. I think what you mean to say is that when he showed up for arraignment there was an additional charge if I'm mistaken please comment and we can try to discuss it further.
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I agree with Attorney Rioux. The officer will write a ticket/summons for what the officer believes was committed. The officer will then forward their report to the prosecutor who will review the report. If the prosecutor believes that additional charges can be proven beyond all reasonable they might add those charges. Additionally, if the prosecutor believes what the officer ticketed/summonsed for cannot be proven beyond all reasonable doubt they will not make the charge. All felonies under the Maine Constitution must be presented the Grand Jury. The Grand Jury will return an indictment charging those offenses that the Grand Jury believes their is probable cause to charge. Again, these charge may differ from what the officer ticketed/summonsed for. Good luck.
My response to your question is a generic response and should not be construed as controlling to your case. I can not effectively advise about your case without knowing all the facts. Additionally, my response does not create an attorney-client relationship. You can contact my office to schedule an appointment if you would like to have me represent you.
Yes, as Attorneys Rioux and LeBrasseur point out, the D.A. chooses what charges to bring forward, in court, by complaint or indictment. What the police officer charges in the first place just serves to bring the person who is charged into the court system. The D.A.s have to actually prove whatever charges are brought, once the case is filed in court, so they will bring whatever charge or charges they think are appropriate. That might be more or it might be less than what the officer summonsed or arrested the person for.