Law enforcement sources in Washington State tell us police in Auburn are undeterred by the accuser's noncooperation with the investigation. We're told Auburn PD is still gathering evidence and plans to present the case to the prosecutor's office.

TMZ broke the story ... the alleged victim says she couldn't take the pressure of taking on a celebrity, and felt betrayed by police -- so she would not testify against Nelly. While her noncooperation makes the case more challenging, it's not impossible to prosecute.

We're told Auburn cops want to present the case to prosecutors soon -- within the next week.

Nelly's attorney, Scott Rosenblum, tells TMZ he's confident the case will be closed based on the evidence. He adds, "I would expect the police to do their due diligence."

Ultimately, it will be up to prosecutors to decide if Nelly's charged. Bottom line? Although he acted like a weight was lifted off his shoulders this weekend ... he's not off the hook yet.