NEVER agree to talk to cops or take a polygraph. Get a lawyer now and follow legal counsel's advice. Stop talking to the cops. Cops do not interview you to help you, only to screw you, that is their job. If they had evidence on you, you would already be in jail.
BTW, polygraph exams are inadmissible in most courts and are not accurate. They often give false readings. Don't listen to anyone that says you are guilty for not wanting to take one. That sounds like a cop talking.