If special counsel Robert Mueller interviews President Donald Trump under oath in the coming weeks, an interesting question will almost certainly be asked: “Did you try to fire me?”

And that’s when Trump could be in some real trouble, law experts say. If he answers “yes,” it could add weight to a case that the president obstructed justice. If he says “no,” he could be committing perjury.

The New York Times reported Thursday that Trump did indeed order White House counsel Don McGahn last June to fire Mueller, whose team is investigating Russian interference in the 2016 presidential election and any role Trump’s campaign played in it. McGahn refused to tell the Deputy Attorney General Rod Rosenstein — the only person with the authority to fire the special counsel — and said he would resign before doing so. Mueller is said to have discovered the incident during the course of his investigation in recent months.

Trump called the Times story “fake news” on Friday, but the White House and McGahn have not denied the report.

Trump’s attempt to sack Mueller is one of a series of events that could be used to build an obstruction case against the president. There’s still an open question of whether a sitting president can be indicted, but proof of obstruction could pressure Congress to act through impeachment proceedings (though the Republican-led Congress has so far shown little interest in that).

“If he’s talking to Mueller and he lies, that’s a crime,” said Georgetown Law professor and criminal law expert Michael Seidman. “I don’t think he’s going to be indicted for that crime, but it might well be thought of by the House and Senate as an impeachable offense.”

Seidman suspects Trump is still considering firing Mueller, despite the president’s toned-down public criticism of the special counsel after he brought on lawyer Ty Cobb, in July.

“The most frightening thing would be Trump fires Mueller and gets away with it,” Seidman said. “Because then we would have a very different kind of republic than the one we are used to. It would be like Nixon getting away with Watergate.”

The legal bar for an obstruction charge is quite high because a prosecutor must prove “corrupt intent,” a somewhat subjective term that deals with internal motive. But ordering the dismissal of Mueller adds to a pattern of firing, attempting to push aside, or trying to influence those overseeing the Russia investigation. And that pattern could significantly influence Mueller’s questioning, experts say.

“Simply put, Trump’s ordering the firing of Mueller adds significantly to the criminal case for obstruction of justice and to any impeachment case for obstruction and abuse of power,” Ryan Goodman, New York University law professor, wrote Friday.

There are already hints that Mueller’s investigation is revolving more around Trump’s attempts to impede the investigation rather than possible Russian collusion. The Washington Post reported earlier this week that Mueller is focused especially on asking Trump about the firing of then-FBI Director James Comey and then-National Security Adviser Michael Flynn.

Trump’s attempted dismissal of Mueller came just one month after he abruptly fired Comey, in early May. Trump explained that he removed Comey, in part, for “this Russia thing,” as he dubbed it in an NBC interview at the time. Comey had told Congress in March that the bureau was investigating any coordination between Russian officials and the Trump campaign. A memo written by Comey at the time says Trump tried to get him to let up on investigating Michael Flynn’s ties to Russian officials (Flynn pleaded guilty in December to lying to the FBI about his contacts with Russians during the presidential transition).

The investigation’s focus on obstruction has frustrated the president, who feels he had to fight back an investigation that has undermined his presidency almost from the very beginning. “Here's what we'll say, and everybody says: No collusion,” he explained to reporters this week. “There's no collusion. Now they're saying, 'Oh, well, did he fight back?'”

After a short back and forth with ABC News reporter Jon Karl, Trump spelled out his exasperation: “You fight back. Jon — you fight back. 'Oh, it's obstruction.' ”

Trump said this week that he was willing — even eager — to talk to Mueller “under oath.” By Cobb later walked that back and told reporters that Trump is “ready to meet with them, but he'll be guided by the advice of his personal counsel."