You better leave town and get a new identity. It is illegal for a 15 year old to have sex with anyone and the cops will arrest you if anyone complains. You need to see an attorney ASAP. Do not make any stacements concerning your relationship with the girl to anyone. Do. not talk to the cops.
You can be charged with a crime. You probably haven't been arrested yet because they are still investigating. Typically, detectives will work on obtaining physical evidence (I.e. DNA) and any other evidence prior to making the arrest. They will also probably talk to you and see if you will admit to anything. Ultimately, you are facing serious felony charges if arrested and you should know consent will NOT be a defense! Do not talk to anyone further about these allegations, including the police, unless it is an attorney. Do not talk to the girl any further, especially on the phone as the detectives will also try to utilize a "controlled call" where they use the girl to call you and record you admitting to the crime. Please know, the State can still prosecute you even if the girl and her parents do not!
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No it is not ok. Don't admit to anyone, even her, that you had a "normal relationship". Sometimes the police will use controlled calls to make a case against you, where someone you know calls you to talk about the situation and the entire call is recorded without you knowing. DO NOT EVER ADMIT TO HAVING SEX. Call a lawyer ASAP.