If a bank employee appears to have used a customers bank account information to pay a personal bill i.e. cell phone bill of the employee, can you approach that employee to find out what happened or does the anti-tipping reg apply to this situation? What if it's an ex-employee?

What does interviewing an employee to find out what happened have to do with a SAR filing, if that is what you are driving at??

Why don't you let the police conduct the interview?

We're dealing with our local PD and customers local PD saying it's the other PD's job.

You can ask questions of anyone you wish. Your security officer needs to step up to the plate. Your BSA Officer will be involved based on the response offered or declined.