"Although nothing in the Constitution or federal law explicitly says presidents are immune from indictment while they remain in office, the Justice Department's Office of Legal Counsel has asserted that they are. A newly disclosed legal memo from the office of Kenneth Starr, the independent counsel who investigated President Clinton, challenges that analysis."
https://assets.documentcloud.org/documents/3899216/Savage-NYT-FOIA-Starr-memo-presidential.pdf
https://assets.documentcloud.org/documents/3899216/Savage-NYT-FOIA-Starr-memo-presidential.pdf