If you have not been court ordered to appear then there is no legal obligation to appear. Additionally, since it is out of state, there is a legal process that must be followed for them to obligate you to appear.
I agree with attorney keller, i would also suggest posting this in a maryland forum to be safe.
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Without a properly served subpoena in hand, you have no obligation or duty to appear.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.