WASHINGTON — A newfound memo from Kenneth W. Starr’s independent counsel investigation into President Bill Clinton sheds fresh light on a constitutional puzzle that is taking on mounting significance amid the Trump-Russia inquiry: Can a sitting president be indicted?

The 56-page memo, locked in the National Archives for nearly two decades and obtained by The New York Times under the Freedom of Information Act, amounts to the most thorough government-commissioned analysis rejecting a generally held view that presidents are immune from prosecution while in office.

“It is proper, constitutional, and legal for a federal grand jury to indict a sitting president for serious criminal acts that are not part of, and are contrary to, the president’s official duties,” the Starr office memo concludes. “In this country, no one, even President Clinton, is above the law.”

Mr. Starr assigned Ronald Rotunda, a prominent conservative professor of constitutional law and ethics whom Mr. Starr hired as a consultant on his legal team, to write the memo in spring 1998 after deputies advised him that they had gathered enough evidence to ask a grand jury to indict Mr. Clinton, the memo shows.