Try reading the damn contempt of congress law. Then feel free to point where the DOJ is given the discretion to present the case to a grand jury.
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What Is a Grand Jury and How Does It Work?
A grand jury is a panel of randomly-selected laypeople that determines whether there is enough evidence in a case to charge someone with a crime.
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A grand jury is a legal body comprised of laypeople that determines whether there is enough evidence to bring criminal charges to trial. During grand jury proceedings, a prosecutor presents an accusation and supporting evidence to the grand jury. The grand jury then decides whether or not the prosecutor can proceed with a criminal trial.