Denying that there was any evidence of his committing an obstruction of justice, President Trump has issued thinly veiled threats to fire Special Counsel Robert Mueller. Like my mentor 45 years ago at Watergate, Archibald Cox, Mueller must take the Department of Justice investigation he heads wherever the relevant facts lead. Like Attorney General Elliot Richardson, Rod Rosenstein is right that, as deputy attorney general, he cannot agree to terminate the special counsel’s investigation without good reason simply because the continuing investigation makes the president uncomfortable or nervous.

There are a number of legal and political reasons why Congress must take seriously the issues of foreign policy and national security involved in Trump’s direct and indirect relations with President Vladimir Putin of Russia. But the clearest reason for a criminal investigation by the Justice Department is more traditional.

Consider just two plausible actions which would, together, have created an unmistakable obstruction of justice. First, General Mike Flynn may have implicated the president, or someone the president wants to protect, in Flynn’s illegal plans for dealing with Russia and/or Turkey. Second, with Flynn facing prosecution for his actions, Trump may have offered a promise of presidential protection as the price of Flynn not trading what he knows in order to obtain immunity or reduced charges from the prosecutors. It may well be to provide this protection that Trump tried to use presidential powers over the director of the FBI in an effort to obtain what Flynn was demanding: the equivalent of a pardon by order of the president (“I hope you can let this go’’). When this didn’t work, his firing the leader of the investigation would constitute a final step to obstruct any investigation even partially dependent on discovering Flynn’s information.

Would Trump be guilty of a significant crime even if only both of the “may haves’’ turn out to be true, irrespective of the Comey firing? Yes, under Section 1510 of the United States Code of Criminal Law, which makes it a crime to endeavor to: “prevent the communication of information relating to a violation of any criminal statute of the United States by any person [such as Flynn] to a criminal investigator.’’

Persuading Flynn to furnish false testimony or to withhold evidence before a criminal investigator or a grand jury would satisfy the requirements of a crime under Section 1510 if the “persuasion’’ involved offering a bribe. Offering the president’s promise of serious efforts to keep the Justice Department from prosecuting Flynn would constitute “anything of value’’ to satisfy the element of bribery in the crime required by Section 1510. Under 18 USC 201 (c)(2), bribery includes promising: “anything of value to any person [Flynn], for or because of the testimony under oath . . . to be given by such person as a witness . . . before any court . . . authorized by the laws of the United States to . . . take testimony.’’

So the next investigative step for any prosecutor would be to obtain any facts likely to throw light on the two possibilities. An investigation focused initially on Flynn’s criminal liability and on possible efforts by him to exact a bribe from the president is likely to reveal the truth. That possibility, without more, requires Mueller to persist until such an investigation is completed. It also explains Rosenstein’s refusal to take part in any presidential effort to fire Mueller without good cause.

What are the indications that both possibilities may well be true? Only they would explain why, despite warnings from then-Acting Attorney General Sally Yates that Flynn had been compromised by the Russians, Flynn was not discharged for several weeks from a critically important national security position with access to our most secret information. They would also tell us why Trump would press for assurance that he could rely on the FBI director’s loyalty — a question no other president has thought it appropriate to ask. And they would also explain why the president would later ask Comey in a secret meeting to provide the rough equivalent of a pardon in the form of going easy in the investigation of Flynn. Finally, they would explain why Trump found it necessary to take the risk of firing an FBI director widely admired in the field of law enforcement under the pretext, later refuted by the president himself, that he was simply following the reasoning he had elicited from the deputy attorney general, whom he hardly knew.

That this set of highly unusual actions would be explained by the two possibilities being true does not necessarily make the possibilities themselves true or the president necessarily guilty of obstructing a criminal investigation. It does mean that the strategy of the special counsel’s office has to begin by exploring the incriminating facts necessary to convince Flynn to reveal whatever it was he may have been threatening to tell if not protected by the president. It also means that the special counsel must determine whether the president pressed others to urge the FBI to desist.

If Flynn was, in fact, successfully involved in extracting a promise of presidential influence (anything of value) in exchange for his own silence, the president will have obstructed justice under 18 USC 1510. Obstruction of an investigation by seeking to prevent an associate from revealing what he knows is an entirely familiar form of crime in the United States. It is not a stretch of prosecutorial interest and power.

Philip Heymann, professor emeritus at Harvard Law School, was assistant attorney general of the United States during the Carter administration and deputy attorney general during the Clinton administration. He also was a former associate prosecutor and consultant to the Watergate Special Force.