"...we are bound by ethical and legal responsibilities to proceed only when there is sufficient evidence.”

If the purpose of a grand jury is to determine if there is probable cause to believe that a crime has been committed, how can a prosecutor say that he can only proceed to convene a grand jury when there is sufficient evidence beforehand?

"The convening of an investigative grand jury would not be productive if the witnesses are not cooperative. Witnesses have a right to not incriminate themselves and cannot be compelled to testify."

I thought transcripts of grand jury hearings are kept SECRET so that witnesses will not be afraid to testify truthfully?