Only one person at the Justice Department has the ability to stop special counsel Robert Mueller’s Russia investigation. But what happens if that person is fired? CNN has reported that President Donald Trump is ruminating about axing Mueller’s boss, Deputy Attorney General Rod Rosenstein, as a way to curtail the investigation. What Trump does next could trigger any number of messy legal battles. Here’s what to know. 1. There is a succession plan Rosenstein, the No. 2 official at the department, is supervising the Mueller probe because Attorney General Jeff Sessions recused himself from all matters related to the 2016 presidential campaign. So if Rosenstein is fired or recuses himself (as a witness to key events), someone else at the Justice Department will have to manage the investigation. This is where the department’s succession plan and the President’s executive order for vacancies come into play. The following list of individuals – which is a mashup of officials because certain Trump nominees have not yet been Senate-confirmed – would be next in line to step in Rosenstein’s shoes: Under this hypothetical, Francisco would be the first in line to supervise Mueller. But Rosenstein’s acting principal associate deputy, Ed O’Callaghan, would become acting deputy attorney general for all other regular duties. 2. Who fills vacancies? The issue of who runs the Mueller investigation could become complicated if Trump tries to appoint someone else to serve as acting deputy attorney general under the Federal Vacancies Reform Act. The President has already done this for acting heads of the Veterans Affairs Department and the Consumer Financial Protection Bureau. But the key question is whether that new person, once appointed, could fire Mueller. Legal experts disagree on this and point to several unresolved issues. The Federal Vacancies Reform Act is silent about what happens when someone is fired, as opposed to resigning. Former Veterans Affairs Secretary David Shulkin repeatedly insisted that he had been fired, whereas the White House said he had resigned. Some legal experts believe that if Rosenstein were fired, Trump could put in whomever he wants because the Federal Vacancies Reform Act implicitly includes firings, it would be impossible in certain cases to distinguish between a forced resignation and a termination, and the President’s hands shouldn’t be tied. Others say, however, that there’s a policy argument that allowing the act to cover firings permits the president to work around Senate confirmation in a way that shouldn’t be encouraged. Still other experts emphasize that Rosenstein is not supervising Mueller in his role as deputy attorney general, he’s doing it as the acting attorney general. Under the special counsel regulations, the “Attorney General, or in cases in which the Attorney General is recused, the Acting Attorney General, will appoint a Special Counsel when he or she determines that criminal investigation of a person or matter is warranted.” Therefore, those experts argue that only Francisco (or the next in line) could step in to fire Mueller because he’s the next in succession to be acting attorney general. They point to the fact that the regulations do not say “Attorney General or Deputy Attorney General.” Yet others argue that by someone else being appointed as acting deputy attorney general, that person should take on all the current responsibilities of the permanent one, which naturally includes the Mueller investigation. Bottom line: This would all be uncharted territory, none of this has been resolved by the Supreme Court and any moves could trigger swift litigation.