You never have to talk to the police. They only way you can be forced to talk about something you witnessed is if you are a subpeoned to court as a witness. If that happens you can hire an attoreny to make a motion to quash the subpeona inorder to convince a judge to withdraw the subpeona.
Luke J Nichols
Spectrum Legal Defense
You should contact an attorney. To answer your main question. The police cannot force you to talk to them that being said you may be served with a court subpoena in the future which is a court order that you cannot ignore.