Did DCF open a case? if not, you would begin the process by contacting DCF. (Sounds like law enforcement is already investigating) understand that there are limits to how many times a child can be interviewed. You will also want to speak to a lawyer once DCF opens a case.
Free Consultations can be made by calling 407-617-1064. Please understand that the information given is not to be construed as legal advice. More information would be needed in order to make more accurate legal determinations on your matter. Furthermore, an attorney-client relationship does not begin until a retainer agreement has been signed by the attorney and client.
It is good that you have tried contacting law enforcement and I'm sorry that it didn't work out with them. I would also suggest contact the Department of Children and Families to report the suspected abuse. Call DCF at 1-800-962-2873. Your niece's mother can also file for an injunction for sexual violence against the perpetrator on behalf of the niece. A lawyer can help with all of this.
This is not to be considered legal advice nor does an attorney-client relationship exist.
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