That’s because Mueller may calculate that if he tries to bring charges in the obstruction case — the part that may hit closest to Trump personally — witnesses may become less cooperative in other parts of the probe, or the president may move to shut it down altogether.

The revelation is a peek into Mueller’s calculations as he proceeds with his many-headed probe, while pressure builds from the president’s advisers and other Republicans to show progress or wrap it up.

The obstruction portion of the probe could likely be completed after several key outstanding interviews, including with the president and his son, Donald Trump Jr. The president’s lawyers have been negotiating with Mueller’s team over such an encounter since late last year. But even if Trump testifies in the coming weeks, Mueller may make a strategic calculation to keep his findings on obstruction secret, according to the current and former U.S. officials, who discussed the strategy on condition of anonymity.

Any clear outcome of the obstruction inquiry could be used against Mueller: Filing charges against Trump or his family could prompt the president to take action to fire him. Publicly clearing Trump of obstruction charges — as the president’s lawyers have requested — could be used by his allies to build pressure for the broader investigation to be shut down.

Other key matters under investigation by Mueller’s team, with its 17 career prosecutors, include whether Trump or any of his associates helped Russia meddle in the 2016 campaign. Mueller is also expected to indict some of those responsible for hacking the Democratic National Committee before the election and publicly leaking stolen material in an effort to hurt Democrat Hillary Clinton.

The timing for whether — and when — to interview Trump or his family members is one of the most sensitive decisions Mueller faces at this stage of his investigation. The special counsel’s office declined to comment for this story.

Trump, who has branded the probe a “witch hunt,” is growing increasingly frustrated as Mueller’s work continues, and the president’s lawyers have signaled that they expect the investigation to wrap up quickly.

Expansive, Aggressive

Recent reports provide a glimpse into how expansive and aggressive Mueller’s investigation is. The New York Times and Washington Post, for example, suggest Muller’s team recently began probing efforts by the United Arab Emirates to influence the Trump team, including a meeting the Gulf kingdom apparently helped organize in the Seychelles where an informal Trump adviser also met with a Russian banker.

The Post also reported that Mueller has been asking about several Russia-related incidents involving longtime Trump lawyer Michael Cohen, including his role in trying to help the Trump Organization build a tower in Moscow in 2015.

When it comes to the obstruction portion of the investigation, Mueller is said to be focused on three main episodes: Trump’s firing of FBI Director James Comey last May; the drafting of a misleading statement about the purpose of a June 2016 meeting between Don Jr., Trump’s son-in-law Jared Kushner and a group of Russians at Trump Tower; and the disclosure that Trump considered firing Mueller last June.

Mueller’s team of FBI agents and prosecutors has already interviewed people who could provide firsthand knowledge of possible obstruction of justice, including Comey, Attorney General Jeff Sessions, Deputy Attorney General Rod Rosenstein, Director of National Intelligence Dan Coats and National Security Agency Director Michael Rogers.

Millions of Pages

Mueller also has turned Trump’s former national security adviser, Michael Flynn, into a cooperating witness. He has interviewed more than four dozen White House and campaign aides and requested more than 1.4 million pages of documents, according to Trump’s lawyers.

Kushner spoke to Mueller early on in the investigation for a limited interview, while Trump Jr. and Ivanka Trump, the president’s daughter who’s also an adviser, have yet to be interviewed, according to a person familiar with the investigation.

There is also no indication that Mueller has yet interviewed Trump’s former bodyguard Keith Schiller, who was at Trump’s side on a trip to Moscow and during each day of the campaign and his presidency until he resigned over the summer. When Trump moved to fire Comey, Schiller hand-delivered the note to the FBI.

Here are three of the main episodes Mueller is investigating on a potential obstruction of justice:

Weekend in Bedminster

One focus for Mueller is a rainy weekend that Trump and top aides spent holed up at the Trump National Golf Club in Bedminster, New Jersey, shortly before Comey was fired. Trump stayed out of public view the entire weekend. While the White House released little information about his activities except to say he had “meetings and calls,” it’s clear Trump and his advisers were discussing Comey’s fate.

Traveling with Trump that weekend was then-Deputy National Security Adviser K.T. McFarland, a Flynn ally; senior adviser Stephen Miller; then-White House communications director Hope Hicks, social media director Dan Scavino; Kushner; and Ivanka Trump.

That Sunday morning in Bedminster, Trump publicly aired his frustrations on the Russia probe, tweeting: “Why did the Obama Administration start an investigation into the Trump Campaign (with zero proof of wrongdoing) long before the Election in November? Wanted to discredit so Crooked H would win. Unprecedented. Bigger than Watergate! Plus, Obama did NOTHING about Russian meddling.”

Late that day, Trump flew back to Washington with staff and his daughter and son-in-law. After landing at Andrews Air Force Base, he spent more than 40 minutes on the tarmac before deplaning, an unusually long time. When he did exit Air Force One, Kushner told reporters “everything is good, he was working on something.”

Less than 12 hours later, Trump woke the next morning and began venting again about the investigation. He wrote six Twitter posts attacking the probe, three of them aimed at former acting Attorney General Sally Yates. “The Russia-Trump collusion story is a total hoax, when will this taxpayer funded charade end?” one of them read. Trump fired Comey the next day.

Don Jr.’s Statement

Mueller also has been prodding witnesses about the crafting of the misleading statement by Trump Jr. to the New York Times concerning the meeting with Russians at Trump Tower in Manhattan. Mueller interviewed the former spokesman for Trump’s legal team, Mark Corallo, after the release of author Michael Wolff’s book Fire and Fury, which said Corallo believed the crafting of the statement could amount to obstruction of justice.

The statement — crafted aboard Air Force One by Trump, Hicks and Kushner spokesman Josh Raffel and relayed to Trump Jr. — portrayed the meeting as being mostly about Russian adoptions. Emails later released by Trump Jr. showed an organizer told him the Russians would produce damaging information on Clinton. The White House has said of the statement that Trump “weighed in, as any father would, based on the limited information that he had,” while on a return trip from Germany.

Corallo told investigators that in a phone call with Hicks and Trump raising concern about the statement Hicks insisted the emails would “never get out,” which Corallo found deeply naive, according to a person familiar with the investigation.

Firing Mueller

The third main area Mueller is probing for potential obstruction involves Trump’s desire to fire Mueller. Trump became enraged with Sessions when he recused himself from anything to do with the Russia investigation, a decision that continues to rankle the president. After Trump fired Comey, Rosenstein appointed Mueller as special counsel.

Trump wanted to fire Mueller in June, three people familiar with the matter said, raising concern among his top aides and closest supporters that Trump would put himself in legal jeopardy. Trump ultimately relented after White House Counsel Don McGahn refused to carry out the order and made clear he’d resign rather than acquiesce.