Mueller’s power is not unlimited. While Justice Department rules say he is not under the “day-to-day supervision” of Rosenstein, the deputy attorney general can still request an explanation for any prosecutorial or investigative act Mueller undertakes. If Rosenstein finds the step “inappropriate or unwarranted under established [Justice] Department practices,” he can order Mueller to not pursue it. Rosenstein must provide a written explanation to Congress if he issues such an order.

And unlike those who once led the independence counsel’s office, a post-Watergate creation that bedeviled the Reagan and Clinton administrations before its abolition in 1999, Mueller could still theoretically be fired by Rosenstein. An unwarranted dismissal, however, would almost certainly trigger a political crisis equal to or greater than the one that followed President Trump’s stunning ouster of former FBI Director James Comey last week.

The move signals the seriousness of the ongoing federal investigation into Russian interference, which first began in July 2016. The U.S. intelligence community concluded in December that Russian President Vladimir Putin’s government orchestrated the cyber-theft of sensitive documents from the Democratic National Committee and the emails of John Podesta, a top aide to Hillary Clinton’s presidential campaign, in an attempt to undermine her bid for the White House. The appointment is also a significant concession to Democratic legislators who had grown increasingly forceful in their demands for an independent prosecutor to oversee the investigation.

Rosenstein’s announcement also underscores the precarious political position of the Trump administration after Comey’s firing. The former FBI director had been dismissed, the White House initially said, over his public remarks during the election regarding the FBI investigation into Clinton’s handling of classified information as secretary of state.

But Trump undercut his administration’s rationale for the firing almost immediately by linking it to the Russia investigation in an interview with NBC News. The New York Times subsequently reported Tuesday that contemporaneous notes drafted by Comey in February subsequently alleged the president asked him to drop the investigation into former National Security Advisor Michael Flynn, who resigned in February after lying about his conversations with Russian Ambassador Sergey Kislyak.

Mueller, Comey’s immediate predecessor at the FBI, previously served as a federal prosecutor in the Clinton administration. George W. Bush nominated him to take over the bureau in 2001. His Senate confirmation came only days before the September 11 attacks, and Mueller oversaw its pivot towards the counterterrorism cases that now occupy most of its caseload. Barack Obama asked him to stay an extra two years beyond the end of his ten-year term, and he retired in 2013.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.