In re: Misc. 4281
Court of Special Appeals, Leahy, Dec. 2, 2016,
Fifth Amendment – A Grand Jury may subpoena a public agency’s internal investigative reports containing coerced self-incriminating statements of its employees, even where those statements may not be used at trial.

So… there’s are cases on point and you don’t get there until dragging the reader through 21 mind-numbing pages of history???
PG County Grand Jury investigating an assault subpoenaed compelled statements made to internal Fire Department investigators by Fire Fighters. The County moved to quash and the Circuit Court granted the motion. The CoSA held that the subpoena was valid, though the evidence likely couldn’t be used at trial.

“[a] grand jury proceeding is not an adversary hearing in which the guilt or innocence of the accused is adjudicated. Rather, it is an ex parte investigation to determine whether a crime has been committed and whether criminal proceedings should be instituted against any person.”

The Grand Jury is prohibited from violating the 5th Amendment, but is not barred from considering tainted evidence.

Supreme Court in United States v. Calandra: “Of course, the grand jury’s subpoena power is not unlimited. It may consider incompetent evidence, but it may not itself violate a valid privilege, whether established by the Constitution, statutes, or the common law.”

Under Garrity, when a public official is given a choice between giving a statement and being terminated, their compelled statement is inadmissible in a criminal proceeding against them. (“We conclude that policemen, like teachers and lawyers, are not relegated to a watered-down version of constitutional rights.”)