Washington (CNN) Rudy Giuliani's assertion to CNN this week that President Donald Trump can't be indicted by the special counsel, and thus can't face a subpoena, banks on a series of internal Justice Department policies.

The question to this day is untested in the court system. Yet the step-by-step process Robert Mueller or any special counsel could follow for a President under investigation has several possible outcomes.

According to several legal experts, historical memos and court filings, this is how the Justice Department's decision-making on whether to indict a sitting president could play out:

First, there must be suspicion or allegations of a crime. Did the President do something criminally wrong? If the answer is no, there would be no investigation.

But if the answer is maybe, that puts federal investigators on the pursuit. If they find nothing, Justice Department guidelines say they'd still need to address their investigation in a report summarizing their findings.

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