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Dear JACUSTOMER - I will assume that the person killed was her boyfriend and not another driver or pedestrian. Your letter should concentrate on her past record, things such as school grades or achievements and any other positive aspects of her life. Judges do no want to hear excuses for the current situation such as "not knowing the boyfriend was in the back". The judge will simply react to those statements with "the driver is responsible for the vehicle".
I know this is a very difficult time for you and your family and for the family of the deceased but taking responsibility for her actions is the best approach to convince a judge that she truly is remorseful and is not blaming someone else or excusing it for some reason.
You can express you and your family's love for your daughter and your confidence that this incident will make her a more responsible person but if you say anything that appears you are attempting to excuse her actions the judge will ignore your letter.I am not permitted to prepare documents from this web site and I can't write the letter since I don't know your daughter and it certainly wouldn't be a letter from the heart.
I'm sure you will be able to express yourself and your support for your daughter. Just do not try to excuse her actions.