President Trump is reportedly apoplectic over the possibility that Special Counsel Robert Mueller might look into his finances—specifically his tax returns—as part of Mueller’s inquiry into Russian interference in the 2016 election. Trump suggested in a New York Times interview that would constitute a possible “violation,” and according to the The Washington Post the president is already looking into whether he can pardon associates, family members, and himself.

But there are no legal or ethical reasons for Mueller to turn away if, during his investigation, he discovers crimes that are unrelated to the original inquiry.

“Mueller is the Department of Justice for the purposes of this investigation," said John Q. Barrett, a former assistant counsel in the special prosecutor’s office during the Iran-Contra affair who is now a law professor at St. John’s University.

Mueller was appointed by Deputy Attorney General Rod Rosenstein after Trump fired then-FBI Director James Comey. Trump later acknowledged publicly that Comey was fired because of the federal investigation into whether there was collusion between the Trump campaign and what U.S. intelligence agencies have said was a Russian effort to help Trump win the 2016 election. Rosenstein, who had recommended Comey’s dismissal based on Comey’s public handling of the investigation into Hillary Clinton’s private email server, made the call to appoint Mueller because Attorney General Jeff Sessions had recused himself from the Russia investigation after reports revealed he had misled the Senate about the extent of his contacts with Russian officials during the 2016 campaign.

Crucially, the order written by Rosenstein establishing the special-counsel investigation is very broad. It states that the special counsel is “authorized to conduct the investigation” that  includes “any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump” as well as “any matters that arose or may arise directly from the investigation.”

“The special counsel has to look into matters under his jurisdiction,” said Neal Katyal, a former acting solicitor general under the Obama administration and Georgetown Law professor who wrote the special-counsel regulations. “So if Trump family financial matters are relevant to the Russia investigation, he would have to look into them. And the regulations do not permit the president to define the scope of the investigation.”

Depending on whether what Mueller discovered was closely related to the original investigation, he might have to consult with Rosenstein before going further. But there are no legal or ethical bars to him pursuing the investigation wherever it leads.

"I think that that's fairly broad as a conferral of jurisdiction," said Barrett. "If in the course of doing investigation A, you find leads or evidence to crime B, ordinary federal law-enforcement jurisdiction is yours. After consultation with supervision, which I guess would be Rosenstein, you expand into the new area, depending on its significance."

This is all assuming that Trump does not attempt to remove Mueller to stifle the investigation. If that happens, it’s possible that the only thing that could revive the inquiry would be an act of Congress, which would require the cooperation of a Republican majority that has thus far been unwilling to directly challenge a president from its own party.