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Mueller testifies on Capitol Hill in Washington Thomson Reuters

Special counsel Robert Mueller is reportedly almost done with the obstruction of justice portion of his investigation into President Donald Trump's ties to Russia.

He might hold off on taking action on the obstruction case until he finishes other portions of his investigation so witnesses will cooperate and so he won't be under pressure to conclude the probe.

Mueller has been gathering information on Trump's possible obstruction since last year, but still needs to interview Trump himself on the matter.

Special counsel Robert Mueller is reportedly almost finished putting together his obstruction of justice case on President Donald Trump, but might wait to file charges until he finishes the other parts of his probe, Bloomberg reported on Monday.

Mueller has been building his obstruction case for almost a year, but he has a host of reasons to want to avoid taking action on his findings until much later. If he files charges now, he is likely to face pressure from Trump and his allies to conclude the investigation, and witnesses may be less cooperative with other elements of his probe.

According to current and former US officials who spoke to Bloomberg, regardless of what he uncovers in the obstruction case, Mueller might make it a priority to keep his findings secret as he continues with other portions of his investigation.

Mueller will likely finish this portion of his inquiry shortly after he interviews Trump himself and his son, Donald Trump, Jr.

Although Trump and his legal team have come up with a multitude of strategies to avoid an interview with Mueller over the past few months, legal experts like Robert Ray, who was one of the independent counsels in the President Bill Clinton investigation in the 1990s, said Trump will have to sit down with the investigators sooner or later.

"The sooner they make the president available to submit to an interview, the faster that Bob Mueller can get to the finish line and be over and done," Ray told the Wall Street Journal last month.

Even Sam Nunberg, a former Trump campaign aide who had a public meltdown on a number of news networks last week — when he pledged not to cooperate with a subpoena Mueller sent him, then acquiesced — said Trump should sit down for an in-person interview.

"The president has to do an interview, I would say. I would highly suggest that he does," Nunberg said on MSNBC Sunday.

What evidence is there for an obstruction case?

James Comey and Robert Mueller walk together outside the White House. Win McNamee/Getty

Mueller has been gathering evidence about the obstruction portion of his investigation since Trump's fired James Comey as FBI director in May 2017.

At first, the White House said it was because of the way Comey handled the investigation into former Secretary of State Hillary Clinton's private email server, but Trump later said "this Russia thing" was on his mind when he made the decision.

Trump also reportedly decided well before Deputy Attorney General Rod Rosenstein wrote a memo recommending he fire Comey on the basis of the Clinton investigation, which the White House released as explanation.

Mueller's team reportedly interviewed Comey late last year, and investigators dove deep into the contemporaneous memos he had written that documented his various encounters with Trump before the president fired him.

In one of these notes, Comey wrote that Trump had asked him to ease off investigating his former national security adviser Michael Flynn, who at the time was suspected of lying about his contacts with Russians during the campaign.

Comey reportedly took Trump's words as an order to stop the Flynn investigation, and while he did not ultimately comply with it, he didn't directly rebuff Trump's request either.

Attorney General Jeff Sessions, as head of the Department of Justice and Rosenstein's boss, was directly involved in Comey's dismissal, and had apparently advocated for it.

In the days leading up to the firing, one of Sessions' aides reportedly asked a congressional staffer if they had any damaging information on Comey, according to The New York Times.

Sessions has disputed this account. He eventually recused himself from the Russia investigation, much to Trump's chagrin.

News has also emerged that Trump attempted to fire Mueller last year, and only stopped when White House counsel Donald McGahn threatened to quit if he did so.

Trump has apparently been asking witnesses who testified before Mueller's panel what they told him, potentially adding to the list of actions Trump has taken that might constitute obstruction of justice, according to The Times.

Mueller is reportedly aware of the conversations Trump has had with these witnesses, and is looking into them.