Who Mueller reports to is of major consequence. Jeff Sessions, the former attorney general, had recused himself from oversight of the federal Russia investigation, so Mueller reported to his deputy, Rod Rosenstein. Now that Trump has fired Sessions, those oversight responsibilities fall to his replacement, Whitaker, who has previously expressed skepticism about the scope of Mueller’s probe.

And in that role, as Dreeben explained in court, Whitaker has significant power: While Mueller’s team is “independent on a day-to-day basis,” if the acting attorney general found anything to be “inappropriate” or “unwarranted,” he could intervene. Indeed, the special-counsel guidelines allow the acting attorney general to overrule any “investigative or procedural step” proposed by Mueller if the move is deemed “inappropriate or unwarranted under established department practices.”

It’s unknown whether Whitaker would shut down Mueller’s investigation if the president asked. But Dreeben’s explanation suggests that Whitaker need not fire Mueller in order to stymie his work. Whitaker wrote last year that the Mueller inquiry had “gone too far” and opined on CNN about the ability of a potential Sessions replacement to grind the investigation almost to a halt. He could opt for a death-by-a-thousand-cuts approach instead of risking the inevitable political blowback from firing Mueller directly. And he could try to hinder Mueller from revealing his findings without having to justify his decision to Congress until after the probe is over.

“According to the regulations, should the attorney general determine that an action is so inappropriate that it must not be pursued, he or she has to report that to Congress along with the justification,” several national-security–law experts wrote in Lawfare earlier this week. “Such a report, however, is not required until the ‘conclusion of the Special Counsel’s investigation,’ so this oversight protection is unlikely to be helpful in the short term.

“Put simply,” they continued, “if someone in Whitaker’s new role wants to create big problems for Mueller, he has ample tools to do so.”

The D.C. circuit court is now examining what influence, if any, Sessions’s ouster and Whitaker’s appointment could have on the Miller case. It is possible that the court will decide that Rosenstein, not Whitaker, is Mueller’s rightful boss, according to Neal Katyal, a former acting solicitor general under President Barack Obama.

Short of that conclusion, however, Mueller may have some recourse in the event that Whitaker maintains control over the investigation and attempts to either suppress it or shut it down. Several legal experts have argued that Trump’s appointment of Whitaker may have been unconstitutional. At issue is the same question of who qualifies as a principal officer. Because Whitaker reports directly to the president, he is a principal officer, these experts say, and would have required Senate confirmation.