Far from wrapping up his investigation, as Donald Trump’s lawyers hoped against hope he might do last Thanksgiving, Robert Mueller may be delaying indictments in his obstruction of justice case in order to bring broader, more damning charges of collusion. For the last several months, Mueller has been asking witnesses about three key events in the Russia probe: Trump’s decision to fire James Comey, his involvement in the drafting of a misleading statement about his son’s meeting with a Russian lawyer in Trump Tower, and his efforts to fire Mueller last summer. As Bloomberg reports, there are only a handful of people left for Mueller to interview, including Ivanka Trump and Don Jr., Trump’s former body man Keith Schiller, and, if he’ll go willingly, the president himself.

Mueller, however, appears to be slow-walking that portion of the investigation, and for good reason. Current and former U.S. officials told Bloomberg that Mueller is concerned that an obstruction case would be politicized by Trump and his allies to hurt the ongoing F.B.I. probe. It would be harder to prove without first securing evidence of an underlying crime. Future witnesses may prove more reluctant to speak with him. And, of course, there’s always the chance that Trump will buck the reins—as he has been wont to do of late—and fire Mueller altogether. Conversely, the special counsel reportedly fears that publicly clearing Trump of obstruction could have a similar effect, giving Republicans cover to call for the probe to be shut down.

So far, Mueller has successfully tread a delicate line in pursuing the case. As special counsel, he was given a broad mandate to investigate any wrongdoing discovered in the course of his inquiry into Russian interference in the 2016 election. As we know, that inquiry has already strayed far afield, sweeping up potential targets from Moscow to Virginia to the United Arab Emirates. Just last week, reports emerged that Mueller is probing a mysterious island rendezvous involving a Middle East fixer, an Emirati prince, and the founder of Blackwater, who reportedly discussed setting up a backchannel between the incoming Trump administration and Putin. And after indicting 13 Russian nationals earlier this year for conspiracy and fraud, Mueller is still expected to bring charges against those responsible for hacking Democratic National Committee e-mails, as well as the e-mail account of Hillary Clinton campaign chairman John Podesta.

Yet there appears to be a strategy to Mueller’s order of operations. By indicting the Russians first, Mueller insulated his investigation from accusations of bias, undercutting the Trumpworld claim that the probe was orchestrated by political enemies of the president. Republicans were relieved to see only foreign agents targeted and cheered the charges, bolstering Mueller’s position. As William Jeffress, a D.C.-based white-collar defense attorney told me earlier this year, the indictments against the Russians made it “a lot harder for anyone to say that Mueller is on a witch hunt,” as they demonstrated that the special counsel “has obviously worked very hard and collected a lot of evidence and brought what seem to be some very credible charges having to deal with meddling by Russian nationals in the campaign.”

It is impossible to know exactly what is going on inside Mueller’s virtually leak-free investigative unit. Still, what we’ve seen suggests the special counsel is moving methodically toward Trump’s inner circle. So far, Mueller has primarily focused on indicting Russians (who will likely never set foot in America) and people accused of potentially unrelated crimes (Rick Gates, George Papadopoulos, Mike Flynn), giving the White House room to breathe. If Mueller has a bombshell, he’ll presumably wait to drop it until he has the evidence necessary to make his case undeniable. By that time, too, the political calculus may have changed—if Democrats take back the House in November, Mueller’s next chess move would face fewer headwinds. And, potentially, impel Congress to bring charges of its own.