Impaneling a grand jury indicates that we seek evidence of criminal activity. Whether a viable case is there or not, is determined by that panel after hearing testimony and evidence. The jury's responsibility is to release a "true bill" or no bill after hearing from those involved in the case.
I think it's more would be more accurate to say that it is believed that wrongdoing has been committed and a empanelling a grand jury empanels authorizes the investigators to issue the formal subpoenas requesting information that may or may not prove it.